Conventional wisdom for any reptile enclosure is, "As big as you can afford, but at least..."
I would keep the depth to 24"; 30" if the enclosure will sit on a typical 30"W folding table.
The reason is anything deeper than 24" will be difficult to reach into to clean, water plants, rearrange plants & branches, etc.
Aside from standard or bioactive, other considerations include all-screen or hybrid, frame material (wood, aluminum, PVC, other), and drainage.
The biggest advantage of DIY (IMO) is that you can get
exactly what you want.
I would spend some time looking at enclosures others have built for things you like and/or don't like. There are 3 Enclosure Pictures threads at the top of this forum, and many, many on
google &
Youtube.