I'm usually in the 210-220 lb range and I've ridden 700x23 and 700x25 tires. I decided I would switch to 700x28 so I bought some Gatorskins. They are much bigger than my older 700x25 tires. IOW all tires are not equal, some will be significantly larger or smaller than others.

As for measuring your rim- you can do this for an estimate. I have an old road rim in the garage, it measures about 19.5mm wide on the outside and just over 14mm wide on the inside therefore each side of the rim is between 2.5mm and 3mm thick.