Just quickly popping in, so I don't have time for a detailed answer.
[some discussion on this thread:
You know what really impresses me?
and here:
Wheel and tire options
I'll post something more detailed when I get a chance, but here's a few pointers:
1) First, every tire has a load rating. This load rating in relation to the actual operating load of the tire is key in determining a decent tire size. Generally, the less of the max loading you use, the better the traction, with the obvious trade off of:
2) Larger sizes = more weight. Not just unsprung weight, which can be hard to manage, but also rotational inertia which can magnify the amount of weight you add (10 lbs on the tires can be the equivalent of 50 lbs added to the body). This goes for both width and diameter, but diameter (adding those dubs) is worse for magnifying weight through increased inertia.
3) It depends on what you are trying to do, but "GENERALLY" the wider you go the more lateral grip you get, and the less steering feedback for the same suspension settings. The taller the sidewall, the "sloppier" the steering will feel, and you'll get a ride benefit as well as pothole/wheel damage protection. (sidewall height is not just aspect ratio, but aspect ratio times the width).
Just some initial food for thought - I'll elaborate with some more tire sizing tidbits... just remember, these are all just guidelines, and there are exceptions for just about every guideline.
That being said, you can basically start with this: Start with the size brakes you think you need. Add about 15%. Then find the smallest diameter wheel that you can put around those brakes without contacting the tie rod (when lock turning) or brake calipers/suspension/etc.
Then, get the widest wheel you can get in that diameter. This width will limit your lower bound for tire width (generally, >235 tire width, you'll need a minimum of 7 1/2 or 8" wheel). Then, if you can afford it, bump the rear tire width (for a near 50/50 weight distribution car) by 10 or 20 mm, so if you have a 235 front, look for a 245 rear to start with. This gives you a bit of cushion in agressive/transient handling (i.e. gives you a bit more understeer).
Then set the aspect ratio so you get about 95 or more for the sidewall height, unless you are fearless and don't care about damaging wheels.
Then check the relation of your tire load rating (that number before the speed rating which corresponds to a max load) to the loading of your car - you want to make sure you are at least as good as, say, the ratio of other cars (like the Honda S2000, I think their front and rear load ratings are 89 or 90).
There are exceptions - for example, you may end up with a BUNCH of extra load rating, which is where the Solstice appears to be. In this case, you may have so much tire that it's OK to have similar sizes front and rear without agressive handling concerns.
OK, now I've gone on too long and I've got to run, but can elaborate with examples if you want...
