The best way is to use a ruler and measure the thickness while you are fully hard. You won't be exact, but you will get a measurement that is close enough. Don't use circumference because most toys use the diameter measurement.

Also, keep in mind that your penis has some give to it, even when hard. Toys with no give (like those made out of metal, glass, wood or hard plastic) feel a lot bigger, even if they have the same diameter. So, if you are using a harder material, go down in size a bit so you don't overwhelm your wife.