I just got my 2004 z about a month ago. I have a set of tires and rims from a different car that fit but are the same size. I noticed the back tires are bigger than the front. Why is that and if i put the same size on will i have problems?

Lots of people run square setups. Won't hurt anything.

Running bigger tyres, within reason, on the back will probably not cause any issues. Having said that, the original set up was designed to keep understeer and/or oversteer within design limits. Larger tyres on the back may result in a change in the extreme steering characteristics of the vehicle. Sorry if this is rather vague as I do not know the sizes of what you are proposing to fit, or the intention behind this ( eg do you want to use the car for drifting etc, or just ordinary road use?)