I have a 15" MBP (the standard 2.4 ghz version) and have the CS3 suite. I use it casually for some apps (ie not for school/intense professional use) and that works just fine for me. You may want to consider upgrading from 2gb of RAM to 4 (after market - it is much cheaper and easy to do. Search the MBP forum for threads on it.) if your computer lage a bit. You can never have too much RAM. That said, there is some natural lag in the CS3 suite, especially with the loading of the apps, so don't confuse that with a need for more RAM.
As for which screen size to get, it depends on your needs and budget. The 15" is fine for me and looks great (I prefer the matte screen to a glossy, but other folks differ). In the 17" you get more screen real estate which you may or may not need. You also have the option of upgrading to a HD screen. It costs more, but again your budget and needs need to dictate that. The 17 ", aside from being bigger than the 15, will also be a bit heavier. Perhaps a consideration for you? One thing not worth the money is the faster processor throughout the line. The stock speed (2.4 ghz on the 15") is more than sufficient. Faster processors will certainly give the computer a bit more speed, but not so much as to justify the higher price IMHO while also draining the batter a bit quicker.
Bottom line, you should go to a local Apple Store if you can and play with the display models. Folks working there should also be helpful and you can get a better idea on size you would be looking for. Also, you may want to check out the MBP forum on this site (vs. the Mac Applications where you posted). It's more active and people there are pretty opinionated. You can also search for which MBP should you buy as there are MANY threads dedicated to this.
