So there are three measurements you need to be concerned with when upgrading wheels. Diameter, width, and bolt pattern. If you go too large in diameter or width, then the wheel and tire will hit either the fender or suspension. You can fix this by rolling fenders and whatnot, but it's easier to just not go crazy large on the wheel. I'm not sure what the biggest you can easily fit on the Capri is, but I suspect a 16" wheel would be safe. Someone here can chime in with more accurate numbers.
The other measurement you need is the bolt pattern. 114.3"x4". Make sure it's that, otherwise the wheel studs won't go through the holes in the wheels.