A player wants a Large character...

[Dausuul]

There is no way to balance it. There are two features that break large characters: 1) double weapon damage dice; 2) an increase in the AoE size of all effects centered on the caster. There is a 3rd issue that will make your life difficult: the way the creature interacts with dungeon environments. Its interactions will always be either too powerful or too weak. If its a wide hallway, large size gives a creature unprecedented control in a dungeon space. They can, for example, block 10 by 10-foot hallways by themselves. On the other hands, in tight spaces, they will constantly be taking disadvantage on everything. These are the reasons the devs have given for an "infraction" on large sized creature design. Instead, in various ways, they suggest that PCs on the larger end of the spectrum should be built at medium creatures with qualities normally belonging to large creatures (such as reach, mechanics that effectively give them bonus hit points, the ability to carry more weight than normal, or the like).

The devs have to design stuff that can slot into any campaign with minimal fuss, including both published adventures and a wide span of homebrew settings. "Can't fit through a 5-foot-wide corridor" is a dealbreaker for them, because it throws a wrench in the works any time an adventure contains such a corridor.

The individual DM, however, can just take account of this during dungeon design. Or they can choose not to take account of it, and throw it back to the player to figure out how to handle the issue.

As for the tactical advantages, a 2nd-level Moon druid can become Large twice per short rest with a 1-hour duration, and I don't see anybody screaming about how OMGBROKEN that is. The only major balance challenge is weapon damage. But weapon damage is the sort of thing that can be adjusted for relatively simply. Large-sized weapons give you +X to DPR; so you need to find a way to apply a penalty of roughly -X. 77IM's solution seems reasonable on paper, and apparently is also working fine in actual gameplay.

 

[Cyber-Dave]

The moon druid cannot cast spells in beast form.

 

[dungeonguru]

Are you planning to do the Hit Die as per p283 of the DMG? Monsters with classes section. A large creature (such as centaur) would add 1d10 per class level instead of the normal progression. This means a centaur wizard would have a lot more hp than an equivalent wizard unless you're dropping some of the feats.

In the end, the question "What does this add to the game?" seems to be the most important question to answer. If it doesn't add a lot of fun for everyone (including the DM), it has a great chance to be a distraction that makes people lose interest.

 

[77IM]

Large Weapons: You can wield Large-sized weapons, but you do not automatically gain proficiency with them. Whenever you would make multiple attacks as part of an Attack action, you make one fewer attack if wielding a Large weapon. At level 5, you gain proficiency with the Large-sized version of any Medium weapon you are proficient with.

I like the idea of this, but I have to point out, that giantish characters at my table don't grow to Large size at level 5 -- they grow to Large size when they give up Extra Attack. A character that never gets Extra Attack won't grow to Large size, and a character that gets it later than level 5 (due to multiclassing, being a Valor bard, etc.) will grow at that later level.

I didn't want to deal with the balance implications of classes designed for only one attack getting a 4d6 attack (e.g. War cleric, Bladesinger wizard with sword cantrips and the ability to cast haste on herself, etc.).

 

[Tonguez]

Medium goes to 8 feet so large starts around 9
I’d just rule no large sized weapons are available unless he makes them himself or gets one as treasure and treat him otherwise as a top-end Medium size