Congratulations! Getting your first bp is so exciting! I would err on the smaller/cozy side, especially for a juvenile. They need to feel secure in order to eat/thrive. Many people use weight or length as a benchmark for when they need to upgrade in size and I've always used a tub until they're ready for a 20 gallon glass enclosure. It's more cost effective and husbandry is also easier during your first year or so. I know they're not the prettiest but it's important to know that you can regulate their environment, especially when you're new to bps. I personally feel a 20 gallon enclosure will be sufficient for at least a year or two (subadult) and remember to provide lots of clutter. I'm upgrading our 1.5 yo to a 40 gallon in the next couple of months just based on weight/length (815g, 38 inches) but he's not uncomfortable in his current 20 gallon. Our 12yo has been in a 40 gallon since he reached adulthood and is doing great!
The most important thing is to provide proper hiding places, temperature, and humidity. Other than that, it's really personal preference! Read as much as you can in the BP Husbandry folder on this forum -- it will save you a lot of stress later. Can't wait to see your new baby!
/chris