Is it rude to ask size?

Not rude. I ask my girls all kinds off random weird questions. They know is wedding related and asking for a reason. They don't care

As long as you ask them all individually and not all together. They may be close to you but they might not be close to the other BM's

Not rude

Not rude

Not rude

Jessica here is an example of one i'm thinking http://oldnavy.gap.com/browse/product.do?cid=1076766&vid=1&pid=120449002 or a similar patterned one from ON. possibly going to have embroidered.

Ask them individually and not as a group.

"Hey (name)! I'm buying some shirts to get ready in for everyone. What size should I order for you? They're button-downs, if it makes a difference. Thanks!"

Nope

Thanks Jay. i said individually in my original question. Thats a given!!

Not rude at all! If you don't mind the shirts not being a surprise, I'd even send them the link. I did this when ordering robes for my bridesmaids, so they could check the store's size chart and let me know which size worked best for them!