I just bought a bicycle, and according to the seller, for me, with 180 cm, I need an L size.
But unfortunately, mine is M. I noticed that only when I'm already having the bike.
Can this do me any harm? Is it safe to ride it?
This is my first bike ever.
A bike frame that is one size too small for you is not going to be measurably less safe than the correctly sized frame. Going significantly smaller or significantly larger can affect you ability to maneuver the bike, (seen most often with children put onto bikes 'they will grow into').
For a small frame, on extremely tight, slow speed turns, you might be 'bunched up' and not be able to turn the bars as much as needed. This should not be unsafe, as you are both going slow, and determine how tight you can make on a bike is a skill you need to ride safely no matter what the frame size. A slightly small frame can increase agility over a larger frame at the expense of efficiency and comfort.
For a large frame, turning tight circles becomes a problem if you have to stretch to hold the bars. This stretch means you cannot balance as well as you might need and cause an unpredictable fall. A large frame may also mean you cannot get the seat low enough, meaning you cannot get a foot on the ground if needed.
While the ideal frame size is needed for comfort and efficiency, one size up or down is usually rideable. Ride a bike that's the wrong size is almost certainly less enjoyable than the correct size.
There are tweaks than can make a slightly small frame fit you better, but you would be much better off seeing if you can swap the bike for a large one. If not, see how it goes and consider selling and replacing with a large bike if you don't enjoy riding it.
It's generally possible to ride a bike that's a tad too small. However, that does not mean that it's equally safe, nor that it will be much fun. There are mostly two issues with bikes that are too small:
A frame that's too short may cause problems for steering (toe strike against the back of the front wheel and/or the handlebar colliding with your legs). This is safety critical.
It may not be possible to put the saddle at the appropriate height. While many new riders tend to ignore proper saddle height, it is crucial for efficient muscle utilization, and thus a huge factor for long-term riding satisfaction.
Proper saddle height means that you get close to fully extending your legs and feet to go through the lowest point of the pedal rotation with your forefoot on the pedal. The knee should remain slightly bent (not locked straight!), but not much so. Since your feet need to extend to go through the pedal rotation, you won't be able to pedal with your heels, nor will you be able to just put your feet down on the ground while remaining in saddle. When you need to stop, you simply need to hop off your saddle.
While this may sound scary to newbies and requires some getting used to, proper saddle height allows you to put much more force on the pedals without overstraining your muscles, allowing your speed to be limited by your cardiovascular system, only. And the faster you can go with the same experienced effort, the more fun you will have riding.
The seatpost of your bike may well be long enough to raise the saddle to the appropriate height, or it may not. You won't know until you have checked (be sure to honor the maximum extension mark imprinted on the seatpost!). If the seatpost is not long enough, you would need to replace it in order to have fun with your bike.
So, these are the two things that I would check before I decide whether to return the bike to the dealer, or not.
When I lived in Tokyo I bought a red BSO from the local equivalent of 1 dollar shop. I was on a tight budget and could not spend much money on a fancy bike, but wanted as much as possible to commute by bike rather than by metro.
I am tall (I usually ride XL frames) and that bike was really small for me: on the ride home I had to watch my knees to avoid hitting the steering bar, even with totally extended seat post.
I then went to a local bike repairer in my neighborhood and asked for a longer saddle stem to have a decent saddle height. It was the first and only time I saw a clerk in a Japanese shop laugh at my request: his face was clearly saying "dude, I can't do miracles".
Nevertheless he gave me the longest stem he had, and with that I rode my bike for the 6 months I was in Tokyo. It was never unsafe, nor I felt complaints coming from my legs.
I also rode a smaller frame for several years when back in my homeland, and had no safety problems.
You won't squeeze out 100% of your juice, but you are not asking about performance.
If I have to say, I find smaller frames to be easier to handle when you stop: you reach the ground much more easily and have less problems balancing on a foot.
When I was in college, I was measured for a 52cm frame (back when they came in individual measurements instead of generic sizes), but purchased a 48cm (road) frame because it was in stock and I didn't want to wait the several months it would have taken to get a new one in from Italy.
I got an extra long seat post and stem as identified by the professional fitting I got (I knew that with a frame that small I'd need longer, the fitting determined exactly how long), then I trained and raced on that frame "safely" for many, many years.
If I put the cranks at horizontal, then turned the front wheel almost completely in the direction opposite the forward crank, the tire would just touch my toe. However, for me, that was an incredibly and completely unnatural thing to do even when turning slowly, and not something that ever impacted my day-to-day riding. I could (and did) track stand regularly on that bike and never had issues with my toe hitting the wheel. It never caused me to lose balance, nor was there ever any unexpected wear & tear on my shoe or tire because of it.
The ride may have been unduly stiff because the frame was so short, but I believe that the long stem & seat post extension may have absorbed a bit of it. However, it was all I knew, so it was what it was.
In short: yes it is perfectly safe (we are talking of a size smaller), no harm, with the use you will discover if it is comfortable or not, start cycling and in a couple of weeks/months you will have a better feeling of your needs.
No, it will be no harm, you need to ride it very intensively to cause harm to your body. And even then, more than the size of the bike it can be a single component (pedal arm, stem, saddle)
On the other hand, a bike one size smaller can be uncomfortable, or as comfortable as the "proper" size, or even more comfortable.
Sizing in bicycle is even more liberal than sizing in clothes. While some guidelines can be derived, the perfect fitting of a bicycle strongly depends on brand, personal preference and body proportions.
Pleas keep in mind that the position itself should not cause any pain, but you will still need some time to break in/get used to the saddle (some bottom pain after half an hour cycling is normal ... in the first weeks). I suggest, after a couple of weeks of use, go to a bike shop and test ride different commuting bikes. Or testing a L or a XL frame of the same bike at the closest Decathlon. You will have a better feeling for bike sizing and posture.
