There are lots of problems with Large size. Here are some raised from the forums:
None of the below prevents you homebrewing a Large PC race. Individually the mechanics balance issues are surmountable, and a Large creature might be fun to play in your own campaigns. However, the size of the list (and number of mechanics fixes that could or should be applied) make it a lot cleaner to start "Medium" and add Large traits such as improved reach using feats.
Mechanic Combat Balance . . .
There are some advantages and disadvantages here. It is actually quite difficult to assess whether a Large creature is "broken". However, it's quite a list of issues for a single trait, which is why the debate goes on.
*** Unfixable ***
You cannot really change these issues without making the creature Medium.
More options for placing Blasts.
More battlefield control due to blocking more squares. This is exploitable with character builds based around marks and opportunity attacks (e.g. a Large Defender).
Easier to grant a flanking bonus to allies, easier to be flanked. This is exploitable with party builds based around combat advantage (e.g. a Large Defender working with a Rogue).
Easier to circumvent enemy cover by choosing which of own spaces to fire from.
*** Fixable ***
Each of these issues can be house-ruled or fixed for a specific creature, but you have to ask: Is it worth it just to have a big token on the board?
Bigger bursts. A Large character generates more squares of bursts and can hit more foes. Fix: Rule that the race generates bursts from a single source square (this is not a perfect fix, but offsets less squares affected with choice of placement)
Better reach. Fix: Rule that the race has Reach 1.
Requires Squeezing in the average dungeon built for and by medium creatures. Fix: House-rule squeezing for the race.
Tougher to Maneuver it is very easy for a Large creature to become locked in position due to lack of possible squares to move to on a typical battle board. Fix: House-rule squeezing for the race.
Campaign Balance . . .
A campaign setting or adventure that made allowances for the Large creature can get around this, but it's a showstopper for anyone expecting a core Large PC race.
Splitting the party. May not be able to accompany Medium characters, unless DM is constantly allowing for the size difference, and ensuring all locations can accomodate the large PC.