I was wandering how I could get more horsepower out of my 97 GTP without any engine mods??

Try a set of headers, a high flow cat and a good cat back exhaust. You can also have the PCM reprogrammed to give you more power. Easiest way without engine mods. You can also change the supercharger pulley, which I recommend after the headers and cat back. This really isn't an engine mod and will give you bunches of additional power.

this may sound stupid, but what size pulley should i put on it

3.4 is the lowest you should usually go, but needs supporting mods. With a 3.4 you should have a colder set of plugs (Autolite 104's are two ranges colder, I believe the 605's are one range colder), 180* thermostat, reprogrammed PCM, a good cold air intake or fenderwell intake (CAI/FWI), and some exhaust mods...I think a better downpipe with a highflow or no cat and Ubend Delete at a minimum. If you do the smaller pulley it's definately worth investing in some sort of scan tool to check for knock retard...basically the lower the better...get too high and you will see significant power loss (due to retard of the spark) and potential for major engine damage (knock causing chipped pistons most often). I run an Aeroforce Interceptor and I recommend it to anyone, it's a great tool and a functional gauge, reads many parameters off the OBDII port in real time.

You CAN go lower than 3.4 but with each size you need more mods to keep your KR in check (that's essentially the biggest problem on modding a supercharged engine). I've seen guys running less than 3.0" but they typically have ported heads, full exhaust systems including headers, intercoolers...etc etc etc. I find it starts off cheap (with 3.4) and gets substantially more expensive smaller the pulley gets.