I know this wasn't exactly the answer you wanted to hear, but modding charater size through brawlbox is more hard and tedious than just editing the t-square.
First, what you need to do is aquire a motion.pac for whoever you're modding.
Next, you need to open the right pannel and open the motion.pac
Lastly, you need to edit each and every motion file.
*note, you have to edit all 3 dimensions, because if you edit 1, the other 2 automatically
change size to compensate.
If you want a quicker fix, just use the partial size modifier codes as demonstrated in this
looooovely video:
watch the tutorial, downlod the stuff and put it through the ocerina cheat manager into the codes folder. Make sure it's named rsbe01 when you put it on your SD card.
Hope this helps!