If you are like me and prefer to watch your snakes and have them as a bit of a center-piece, keep with the glass (or PVC or whichever visual vive); if you want more space to keep more snakes, you can do the racks. Lots of options!
I find a good rule to stick by for size is, if the snake can stretch out completely from one length + one width measurement (i.e. if the snake stretches out across the front and side of the vive) and is no longer, the vive is large enough. I think you can clutter up almost any size vive that may be too insecure/large/uncomfortable for the snake to make it feel less so. This can be done a tonne of ways, including blacking out 3 of the 4 sides with dark paper, adding multiple hides - tissue boxes, plastic containers, whatever - bunched up paper for young ones is something I've heard of lots, etc.
All the best! :)