You will have to get yourself measured - or get someone to help you. You then need to compare your measurements with the measurements on the dress sizes, but please be aware that there are variations with the manufacturers. Some clothes in say, size 10, may fit perfectly from one manufacturer, but a size 10 somewhere else may be too big or too small! I recently ordered a prom dress for my daughter; her measurements did not match with even the smallest size, so I had to pay a bit extra for them to make the dress according to her measurements. You may find that that will be an option for you too.
Hi, Here's a link to a size guide I found. It actually shows you how to measure yourself really easily. Let me know if this helps!
