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
Thank you for the response.
The measurements are just right to fit the dimensions of the tank
The snakes won't be coming into any contact with the pine. I was also planning on treating it with lacquer before using it. Will any of this cause a problem?