Look on the driver's door support and there is a decal with sizes and tire pressures. DO NOT mix tire sizes as it will disrupt the antilock brakes.
Just use the suggested R15
245/50R16 get a W rated tire or Z rated
the spare tire should be on the under carriage under the trunk of the Blazer
There is a sticker on the bottom or side of the drivers door that will tell you exactly the original tire size range for your jeep. It will vary somewhat depending on the exact original equipment but the vehicles speedometer and engine management system are set originally for those sizes.
630 pitch, 96 links
6 1/2
26r 29f
265/75/r15
Look in the tire wall it tells you the tire size "h&w" and rim size
You can purchase a used one from your local salvage yard. This would be easier than building one.
The 1996 General Motors/GMC Sonoma 2WD had the OEM tire sizes of 205/75/15, 215/65/15, and 235/55/16. Aftermarket wheel and tire combinations are offered up to 20 inches, such as with a 255/35/20 tire. It is a safety imperative that any aftermarket wheel/tire combination are safe to handle the load and speed of the vehicle. Additionally speedometer calibration will be needed to avoid bad readings.
Open the driver's door and look for a sticker on the edge of the door or the pillar. That sticker will have the proper tire size on it.