I have a bridesmaid that lives in the US too, so I know where you're coming from.
Think it mainly depends on the style of dress you go for.
I've got skirts and will be getting vest tops to go with them, so it's quite a relaxed look, will look something like this:
Means I just needed dress sizes rather than specific measurements. Also, I got the skirts from Amazon for a bargain!
So you could go for something like that? Or if you want something a bit more formal, my friend ordered her BM dresses from JJ's house and put in the measurements for each dress. They came out really nice. So you would just need to get the measurements in inches or CM, which will be the same wherever they are in the world. The only downside of this, is if they lose or put o weight, you can't return made to measure.
Another option is to buy dresses online based on dress size, and maybe arrange for the girls who live abroad to have theirs sent there? Then they can try on and return for a different size, or bring with them if it fits?
Hope this helps! X