You need access to the spare tire. On the side of the tire, in raised lettering, you will find writing like this: P225/75R15. Or like this: 30x9.5. Or like this: LT25575R16.
Use the chart below to find your tire diameter in inches and purchase the cover that most closely matches the size you have. Round up to the next inch if necessary.
Some tires will actually have the tire size in inches printed for the size: 30x10.5 (30" tire diameter x 10.5" wide - go with a 30" cover)
Every manufacturer is different in the sizes they produce. If you don't see your size, you can order a cover up to an inch larger and it will fit fine. Don't choose a smaller size as it probably won't fit.