Rep Power: 0

How do I know what size touchscreen head unit to order.? I want to order one off of eBay.

 
Rep Power: 256

there fairly universal just don't get some off chinese brand.

 
Rep Power: 195

It based on the opening size of your current head-unit. If it's a single din (about 2" tall) you are limited to what ever motorized kind you can find. Usually around 5".
If it's a double din opening (about 4" tall) you can get anything from 5 to 7".

Go to Crutchfield.com and use their "What fits my car" tool. It will give you all of the ones they offer that fit. You can then use that for a comparison for what's on Ebay.