You could just use the standard size that'll be in your computer manual, if you don't have it will probably be online.
Or if you want to be more accurate you could measure the circumference.
I find it easiest to start on flat surface with the tube valve at the lowest point, mark and push the bike forward until the tube valve goes full circle, mark again and then measure the distance between the 2 marks. Although not neccesary I usually do this a few times.