I've only done it once but plan on buying more. I only looked at people with reputable history and ratings to avoid scams. I had a custom order from a really nice lady and we talked back and forth through messages at least 10 times making sure I was completely satisfied with everything, package was fine and came quicker than I thought it would.
If you have questions like that always ask the seller, I was upfront with my seller and told her right away I wasn't familiar with etsy and apologized and asked her about the things I was confused with. With my custom order we talked about all the details in messages (probably where you'll specify sizes and whatever else).