Now lets be sure everyone understands what is meant by "Dual" application vs "Tandem".
Tandem means one following or behind the other. Most trailers have either two or three axles in "tandem" and each axle has just two tires on it. This utility trailer has tandem axles.
Ok we have definitions behind us. This post will address the proper way to match a pair of tires for dual application. This is very important, as mismatched duals can result in overloading and in extreme cases could lead to tire failure.
If you are buying new tires you should find that all the tires are almost identical in OD when first inflated. If you were to accurately measure the tires after a few hundred miles use you would find that the tires have "grown" or stretched out a bit as they were heat cycled, but again the tires would be very similar in dimensions.
If you were going to try and measure the OD of a tire you would also learn that it takes special tools to get a direct measurement. The best way to measure how big a tire is is to measure the circumference. You can do this with a standard tape measure. You don't even need to calculate OD from the measured circumference as I will give you the guidelines based on circumference. You just need to be sure you keep the tape at the center of the tire.
Here I have affixed one end of a standard tape measure to the center of an inflated tire using masking tape.
I next carefully roll the tire around, keeping the tape taught and in the center.
When the tape comes around I can get a fairly accurate measurement of the circumference. In this example I didn't use the end of the tape as the tang doesn't lay flat, so using the 3" mark I see 95" for an OC of 92.0 inches.
Now you should match tires +/- 3/4" of circumference in dual application.
If placing new tires in dual application you do not need to do this measurement but if trying to match used tires or a new tire with a used tire this matching is important.
If you do not match the tires the larger tire will be forced to carry an increased load and in extreme cases can result in failure.