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According to the Arch Wiki installation guide, the recommended size of the EFI partition is at least 260 MiB. Nevertheless, every dual boot installation guide I saw doesn't even touch the question of the EFI partition's size and uses Windows' 100 MiB one.

At the same time, I've read that there should be exactly one EFI partition on the same disk. Without knowing it, I installed Arch many times creating a separate EFI partition for it and everything was fine. I just had to change the boot order (or use the boot menu) to boot into the needed system.

So what's the right way to install a dual boot system?

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The right way to install on EFI a dual boot system, is for both operating systems to boot from the same EFI partition. This allows the firmware to display the dual-boot menu and simplifies removing one operating system.

If you install dual-boot using two separate EFI partitions, you need to specify to the firmware which partition to boot from. You will not have a dual-boot prompt. Removing an operating system requires knowing which EFI partition to delete (and not making an error).

Not all firmware versions will support booting from more than one EFI partition. Many just search the disk(s) for the first EFI partition that they can find.

The EFI partition itself is quite small, so no point in setting for the minimum size of 100 MB. The maximum size mentioned is 550 MB, which is still quite small for modern disks.
See What is the absolute minimum size a UEFI system partition can be?

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