I was doing some research online and I found out that the diameter of the 95-00 4 runner rotors is 12.6" ...if that's the case I guess 15 inch rims will fit in my 4 runner right? its a '97. or are there other factors, aside from the diameter of the brake rotors, that won't let me put 15" rims?

 

15" Rims will only fit if your truck has the smaller brakes (ie originally came equipped with 15" wheels). The 16" wheel equipped trucks have larger calipers that will hit the wheel

 

It's not the rotors thats an issue, it's the brakes.
There are a few 15" wheels will fit...namely rims with 4-3.75" backspacing, like Cragar "soft 8's" and certain aluminum wheels. It's really hard to determine all the 15" wheels that will clear the brake calipers because hardly anyone has gone this route...I've seen a couple that have though.