This model carried 30x3 1/2 tires. That obsolete marking system starts with the outside diameter of the tire ( not the rim). The smaller number is the tire size, thus 30 minus 3 1/2 on one side and 3 1/2 on the other side leaves 23" as the rim size. In this drawing you will see that the rim size is not the outside diameter of the rim ( edge to edge) as the rim has a flanges on it to keep the tire from coming off. Modern rims are the same, the actual rim size ( inside diameter of the tire) will be about 1 1/2" less than the maximum outside diameter of the rim. A reasonably close place for you to measure the 23" would be the outside of the wood wheel felloe ( inside of the white/silver painted rim).
Hope this makes sense and is helpful.