If you explore clothing online, or travel and shop away from home, you'll soon come across wonderful apparel pieces that just jump out at you and scream: Buy Me!
Just for these instances, there are great dress size and shoe size conversion tables on Wiki that can be invaluable resources.
If you know the range your dress and shoe sizes usually are at home, you can then compare to the other countries. Of course, nothing beats having your measurements at hand, and trying things on in person...but for all those other times, this is a great resource.
And remember, size is just what the manufacturer uses to template their apparel patterns. Each manufacturer's size can range depending on where the clothing was manufactured, how far up or down in the textile the pattern was cut (as the pattern is cut down, things wiggle around). You're not a size, you're a wonderful person with personality, intellect, gifts, and sass--and excellent taste. Wear what fits you well--AND get to know a great tailor in your neighborhood--and work it!