I'm just about 6 foot, wear about an 18 and I'm a DD up top

I have an Ellaroo ring sling in a medium. It works well for me but lately since Reece has gotten a little bigger I've noticed that the tail is too short to cover Reece up when he lays down in the sling to nurse. I could really use a large now for that reason.
Next I have a Moby D. Great, comfortable way to cary a baby but it's a lot of fabric and not something you just toss on quickly. One thing I have found about mine... I wonder if it's too small for me b/c of the way it ties. Hard to explain but I really think it needs to be longer to tie properly.
I also have a baby Trekker. It's kinda like a heavy duty Mei Tai. It works really well for us too.
If you are a size 20 or smaller, you should be able to get away with slings and carriers in large size without a problem. At least that's what I've found to be true myself.
As for slinging twins... I never wore my twins at the same time but I have a friend who has. She liked to put the boys in 1 ring slig for when they were small. When then they got older she liked to wear two slings, crossed them and she put the boys in a hip cary.
Anyway... Hrere are a couple of links for you.
Wearing twins
http://www.thebabywearer.com/articles/HowToO/Twins.htm
Plus sized babywearing
http://www.thebabywearer.com/article.../PlusSizes.htm