for just about any unknown bolt/nut, if you put an adjustable wrench firmly on it, then you can measure the size pretty accurately by measuring the distance between the jaws.
of course you need a good measuring device. the flat metal ones seem to work best. if you use a standard tape measure, then 'burn an inch' which means you dont use the standard bent tab end. you hold the start of the 1 inch mark on the beginning of the measurement and then read the dimension. theres 25.4 mm per 1 inch if you need to convert.