ClassiCube does not have complex block models like modern minecraft, only cubes and sprites
you can change the size of the cube with /blockedit (block) min/max (dimensions), or set the block to a sprite with /blockedit (block) drawmode 5
however, if you're on a server with the Custom Models Plugin, you can create a bot with any block model and that works well as a substitute. it's explained here.
one more thing, if you want the bot to have solid collisions, you can create an invisible block and place it inside the bot
feel free to ask if you have trouble with anything