You have a 1994 mercury topaz what would you have to be done to it before you can put bigger wheels on it?
That would all depend on what size you are wanting to go to, I just got a 1994 GS with a 3.0 L engine. The factory tires on it are P185/70R-14, It is a 4 bolt pattern, and currently I am not sure of the back spacing to it, but will be looking into it shortly. For me, I am looking for putting on larger breaks (hence larger wheels, after all why go larger if its not for a legit reason a.k.a better stopping power) I am probily looking on updating the breaks from a AeroStar mini-van, larger breaks = better stopping power, I am looking at 16 inch rims, that are probily 16" x 8" in size. I know that some of the same cars that came out of Ford run that size as factory and are in that class (Escort, Tempo, EXP, Lynx) I know that putting on P175/60R-16 will only change the speed by .03% = 60Mph by the speedo, your actually going 60.2 Mph. so I will not have to worrie about the Speedo being off enough to get a ticket! The wider tire (hence going from a 70 series tire to a 60 series) will give me a better contact pach to the road and better handling and breaking time. A good tool to use for figuring out tire size is using a tire size calculator, like this one at: http://www.miata.net/garage/tirecalc.HTML
As long as you use your head and not go too far on changes, and you have good enough shocks, and watch your back spacing, it is really a simple change. If you deside to go to something too out there like 50 series, you will not have to change the backspacing from factory, but sometimes it is best to cut the backspacing a bit, an example: on my daytona I have a backspacing of -44mm. backspacing, but I wanted to run P195/60R-14 (found on ford conturs, Daytona runs the same as this Mercury). I went to a -42mm. backspacing for a little protection to keep from rubbing the inner fender wells. The -02mm. difference helped keep that from happening, and the handling improved tremenency!