Help with tires/wheels

I'm going to mod my Hyundai Accent LS 4-door 1995 in the near future, and to be safe I'm gathering information about several stuff here and there.

First of all, please someone confirm that my car (ie. Hyundai Accent LS 4-door 1995) has a maximum wheel size of 15". I read this on the car's manual and searched the internet, and I get the same result on every search, ie. it can handle up to 15" wheels.
 
Second, I need someone to explain to me if I can fit larger rims (ex. 16" instead of 15") by reducing the tire threads, and if this is feasable for bumpy roads and cost.
 
Thank you for your time.

You could fit 16 inch rims if you buy tires with a lower profile. The ride quality would probably be worst.