From the inside. In most cases you round to the nearest number ie; 4-1/8" = 4", OR you can pull the fixture down and look at the sticker on the body.

You need to find out the brand and model number of the housing and what trims are available for it. The housings and trims are UL listed together and for the most part you cannot change brands. Unless your housings are very old the info you need will be inside the housing on the cylinder. Good drywallers and painters protect the inside of the housings and you'll be able to read it.