Unless you have a humongous female, a 20L should be fine for them for their whole life. As long as you can fit two appropriately sized hides and a water bowl, you should be fine.

I use tubs and only have two snakes (Shameless tub plug), so I am not really familiar with the dimensions of tanks vs size of a BP. The biggest tub I would use for an adult BP would be a 32qt which is about 8 gallons. Then again, tubs lack a lot of the height that tanks offer. Based on this, I would assume a 20L would be suitable.

I would be a little wary of that shelving unit though:
Product description & measurements
Post/ shelf:
Solid pine or spruce