It's roughly how tall the tire is from bottom to top. Stock tires are around 27 inches. Shorter tire increases acceleration at the cost of top speed. A 26x10-15 is 26" tall and a 28x10.5-15 is 28" tall. Here's a link to help see the widths and sizes of slicks: Mickey Thompson Performance Tires & Wheels
My 12.236 was done with a 26" (275/50-15) MT ET Street Radial on Racestar 15x8's that don't need any sway bar relocation kit. Stock torque converter, 4.10 gears, all the bolt ons, and taking off at 1000rpms and those tires held good. They made it about 4 trips to the track with 4-6 races per night and another 4000 miles. I'm soon going to be upgrading to a 28x9-15 slick which is 27.3" tall to stay in 4th with the nitrous and help get traction with a 100 shot.
I attached a pic of how a 26" drag radial on 15x8s looks with a 1" drop all around.
2011 Mustang GT Auto
Zex 150-shot, CAI, Hearthrob H-pipe no-resonators, Flowmaster American Thunder axle-back, PSI Motorsports tuned, 4.10s, springs, shocks, struts, sway bars, adjustable panhard bar, adjustable UCA, LCA relocation brackets, LCA, and 275/60-15 MT Radial Pros.
12.236 NA before nitrous
11.719 with nitrous spinning through first.