I find going to the store is the easiest method, although sometimes scary. I spent a lot of time with a measuring tape to get my dimensions figured out to get the right size clothing online, but each manufacturer has a different sizing scale and even within manufacturers, the sizes may differ. If you must go online and don't have a baseline set of clothing from a specific manufacturer that you can use as a guide, I'd suggest going to a trans site to get sizing. The sizing of regular sites it based on the premise that your body type matches that of the gendered clothing. I don't have hips and my stomach is larger than my chest, so they never match up for me. Trans sites have all that stuff taken into account and can more accurately figure out your appropriate other sex sizes based on your current sex measurements.
