Noticed the previous thread, so I'll welcome you on this one. Next for this post and the last, everyone will tell you to get an XCAL II. It's a flasher and I haven't been able to afford one yet but want to do some more mods before I do. WFoy is a site sponsor and knows how to get everything out of the ST with the programs.
Depending on the year of your ST, a couple tire/rim sizes might have come stock with your ST which will matter when you use the tire size calculator that Austin posted above.
Some of the 2001s I believe had 15 inch rims ( can't remember what the tire size was).
Most 2002s and later years either came with 235/70/16 or 255/70r/16.