There were no 15" options for these cars, from 1989 -1992 in the 3G platform. Only 13 and 14", steel and aluminum.
That said, yes you can put 15" and up, in place of the smaller sizes. Galant 15" rims are a popular choice to keep weight down and maintain period correctness.
E39A wheels I have used in conjunction with Galant front rotor made non-captive on stock 3G CSM knuckles, and a spacer set to utilize Wilwoods and maintain a hub centric fitment.
Build the Expo/RVR on the site I list and you should be able to determine the minimum wheel size that the vehicle came with if that isn't already listed in the Expo/Van forum section.
I only see 14 and 15" factory options listed.
The Mitsu's of this era used a fairly high positive offset, as to most FWD vehicles. My Subaru rims push into the +50mm offset, whereas Mitsu had +40 to +46 just from what I have seen recently.
The site I listed should give a general idea on scrub also, as you change the offset of the wheel and tire size, you will change the dynamics of the suspension as it travels straight and as it corners left or right.
Use the comparison tool to get a feel for that part.