if you want to make an individual image inside a sprite sheet bigger, then i'm afraid smbx doesn't let you go above 100x100 by default. if you want to edit the player's hitbox/graphic offsets, you can use "pge_calibrator.exe" located within data/PGE. this will let you generate ini files which you can place in your episode/level folder in order to override those settings.
also, although it would technically be possible to just make the player completely invisible and then use a lua script to manually draw the correct frame which would let you go beyond the 100x100 limit, this would invariably be somewhat difficult to pull off.
Sorry if I didn't explain it well.
Here's an image.
The red square is the GRAPHIC size (I don't mean the hitbox).
Left image: default graphic.
Right image: bigger graphic (what I want).
It will work without problems if I want to increase the graphic size?