Skeetendo

Unfortunately, there is a not a single routine you can add that will allow for 48x48 back sprites, instead of the original 32x32.
There are a few routines that need slight modifications.

Specifically, there are three routines that all need to be modified
LoadMonBackPic at 0f:7103, for player's Pokemon back sprite in battle
LoadPlayerBackPic at 0f:6c92 for the player's back sprite in battle
and HoFLoadPlayerPics at 1c:433e for the player's back sprite in the Hall of Fame

You can read a lot of discussion on this problem here.

Are you hex editing your hack or are you using the disassembly?

I know the thread I linked you to is hard to read, because so much of it refers to addresses.

To make it simple, you have to make a very small function, called LoadBackSpriteUnzoomed, and in the three functions I mentioned above, replace the calls to ScaleSpriteByTwo and InterlaceMergeSpriteBuffers with a call to this new function.

In LoadBackSpriteUnzoomed, the routine ends by jumping into the middle of another function - the function used for loading front sprites, LoadMonFrontSprite.
You can do this by creating a new label in the middle of LoadMonFrontSprite, called LoadUncompressedBackSprite, and jump to that label at the end of LoadBackSpriteUnzoomed.

Here is what I mean. (of course, in LoadBackSpriteUnzoomed, use $66 (or whatever dimensions your back pics are) instead of $44 like I did in this example)