you can actually swap player inventory size real time as long as you call the correct functions on the client every time you do.
this overall seems like a neat concept and i might make it in the future. however it would be a great first mod if you are interested in learning how to mod.
id start with looking up old mods which sorta do what you want to do, eg seven slot playertype. you can find it on any rtb archive; atm the only working ones as far as i know are Blockland Glass, swollow's, and oRBS