Believe what you want...

[mikeinaus]

13" is going to be extreme. the ideal ride/handling rim size for our car is 16". smaller will give you a softer ride but tire roll will increase. not too mention 13" rims will look horrible.

[Inodoro Pereyra]

so by combining stiff springs like H&R's (and a good strut combo) with say 16" wheels vs 17" wheels and the proper tire sizes, the 16" wheel and tire choice is going to ride a lot better than the 17's will because of the increased sidewall of the tire. with 16" wheels.

Ok so with this theory is better to use 13'' than 14'' cause with 13 we will have more sidewall isn't that right?

To a point.
Remember that in mechanics it's all about balance, and everything is a compromise. A 13" wheel will give you more sidewall, that'll make for a more comfortable ride. As a downside, the tire will deform more (sideways) when subjected to a side load (like when taking a curve), and you will have less room in the rim for the brakes, which means you'll have to settle for smaller rotors.
So the best solution is always to look at the whole picture, and make a decision based on your (realistic) expectations.
If you're using coilovers with stiff springs because you want to lower the car to look good, don't use 18" wheels, or your molars will loosen up within a few miles.
If you want to lower the car, but at the same time ride comfort is very important to you, go with a small wheel (or just keep the stock ones).
If you want to go racing regularly, a 16" or 17" will give you enough sidewall to make the ride somewhat bearable, while the tire doesn't deform so much, and at the same time, you will be able to fit a more adequate brake system for the demands of the track.

[Nd4SpdSe]

15' would be lighter weight and give you the best selection of tires (well, used to be, things may of changed on that in the last few years)

[concealer404]

so by combining stiff springs like H&R's (and a good strut combo) with say 16" wheels vs 17" wheels and the proper tire sizes, the 16" wheel and tire choice is going to ride a lot better than the 17's will because of the increased sidewall of the tire. with 16" wheels.

Ok so with this theory is better to use 13'' than 14'' cause with 13 we will have more sidewall isn't that right?

ROFL!!

It'd be better for comfort, worse for performance, because 1) There isn't jack for decent tires in 13" size, (not much for 14" either) and 2) talk about sidewall rollover.