Playing D&D 5e. The Cleric (Tempest domain) in my players' 4th-level party (Cleric, Ranger, Rogue, Warlock) spotted Enlarge/Reduce and has taken a liking to it for hijinks of the Enormous Half-Orc or really tiny halfling.
However, Enlarge/Reduce isn't a Cleric spell. It's only on the Wizard and Sorcerer lists. Is there any balance reason Clerics shouldn't have it, or is it likely to just be flavour?
A single level 2 spell is not going to break your game. But there is a reason why the spell lists for different classes differ, and it is not balance: it gives them distinction, so they play differently and fill different roles, and play is more interesting and encourages a team working together.
Cleric domains already weaken the distinction and provide access to other spell lists. For example the Light domain even gives access to fireball, a signature spell for wizards and sorcerers -- but domains do so to support a theme. The Tempest domain likewise gives access to a lot of classical wizard spells, such as lighting bolt and ice storm -- but all these are thematically related to storms and weather. Enlarge/Reduce is not. You'll not affect balance in a major way by swapping out gust of wind or shatter with it. Shatter is a pretty decent area damage spell at third caster level, and can take out multiple weak foes. Swapping it out for Enlarge/Reduce will not make the character stronger.
This is supported by the DMG, page 287:
Modifying a class's spell list usually has little effect on a character's power but can change the flavor of a class significantly. (...) Be cautious when changing the warlock spell list. Since warlocks regain their spell slots after a short rest, they have the potential to use certain spells more times in a day than other classes do.
So this is more a question of how gonzo and everything goes you like to play. If you and your player are happy with it, go ahead.
The Simic Scientist background from Guildmaster's Guide to Ravnica and the Quadrix Student background from Strixhaven: A Curriculum of Chaos both grant the Enlarge/Reduce spell to characters that have taken them.
Additionally, a Cleric can qualify to take the Adept of Red Robes feat at level 4 if they've taken the Mage of High Sorcery background, from Dragonlance: Shadow of the Dragon Queen.
