Our almost 9-month old has been home for 1 month now from Guatemala. I too thought I would have a 4-6 month old home. Well, we lost our first referral from Guatemala 7 months into it, unfortunately. Then with this adoption we had lots of delays thanks to civil registry, PGN, US Embassy, all out of our and our agency's control so she didn't come home until the end of her 7 months.
I used diaper buying as therapy during the 18-months of our adoption process. It was a nice hobby to keep my mind occupied yet focused on baby things. I didn't think I would do it as a hobby. I started buying diapers a month before I thought our first daughter was coming home. It was nearly a year later until I finally got to use them.
I'm not saying this to scare you. I'm just telling you that Guatemalan adoptions are so variable, as you know, that it can really be painful at times to see the diapers sitting there.
Plus, so many people told me to plan for her to be slightly smaller than US kids. However, our babe is a chunk, and she's 75% on US growth charts for weight and head and 50% for height. I planned mostly on mediums, but I didn't know that we would have fit issues with front vs. side snapping. Her thighs and waist are sized just so that she can't use any front snapping diapers at all. Sooo, for the past couple of weeks I've been reselling all my front snapping diapers.
Honestly, if I were to do it all over again, I would wait until I was out of PGN to buy anything. If you have a GC BC, then there are still 3-4 week delays so you would have plenty of time to buy stuff.
I say that even though some of my favorite things are not available right now. But, it has been kind of a pain to resell all the things that don't fit. I'm getting back some of my money but not nearly all of it.
Just plan on getting some easy-to-get things at first. Speaking from experience, I would skip prefolds. The babies are so wiggly at 6+ months that if she is your first child, you have enough things to figure out and you should focus on bonding so make things as easy as possible. Unless you really want to do this to save money, I would skip them.
The first diapers I bought were Fuzzibunz, and I loved them and I honestly have yet to even test them on her. I thought I was smart buying one-sized diapers, but they all front-snap so none of them fit her.
Even as chunky as she is, she fits well in just about all medium side-snapping diapers.
PM me, and I'll send you some diapers to start you off.
Diapers aren't my hobby anymore; I'm too busy with my girly, but I'd love to help you get started!
ETA: Oh, I forgot to mention that you will typically get a medical report each month with her weight and height. That helped a bit!