You'll want no bigger than a 16mm piston size. 14mm is better. The caliper is only a 2-pot slider which is pretty basic. Braking RS-B1 or MC6601 or MR1601 could work, MotoMaster, Magura 195 series (0120791) make nice replacements you can find in the various distributor catalogs. Shindy products (aka Nissin) has a 14mm configuration (model 17-652), too. If you're going to spend the $200+ on a new MC you might want to look at finding a nice 4-pot caliper to retrofit. Then you can run a 16mm piston no problem. Measure the bolt distance and maybe you'll get lucky. Or have someone make you an adapter plate.
Personally I would focus my energy on pad selection and possible aftermarket rotor options. I commend your enthusiasm and all, but as a 'relatively new' rider, you might want to focus on more important tweaks.