Especially with autocross not being in the plans where you can run afoul of restrictions on wheel and tire sizes, try to peer into the future for sizing them now so you don't later find out that you've limited yourself too much. 11" will give you more room to grow as a driver, and it's beginning to look like much of the better rubber in big enough sizes is in the process of moving up another inch from 18. In 19", you'd probably want to look at tire sizes 5 profile points lower than what you'd look at in 18" for the same width tire.
Norm
New on my study list ---- deciphering and sorta cross-referencing many of the tire sizes by brand per rim size and width along with reviews on performance vs life or best or happy medium or both. And sort slick vs street. It will be a year until I'm on a track. And sell the dog so I can upgrade brakes so I can utilize all of the wonderful contact.