You could probably get away with a 20L sistema tub but I would go with a 30L for a little more height that would make them feel less closed in. My 2 cents worth
How many shelves did you want and how many tubs you gonna put in each shelf and how much weight per tub including hides sand etc make one tub up and weigh it then figure out how much each shelf can hold maybe you can put some geckos that dont dig so much up the top so cause you wont need so much sand
You could probably get away with a 20L sistema tub but I would go with a 30L for a little more height that would make them feel less closed in. My 2 cents worth
They don't look like sistema tubs (at least the bigger one doesn't). Will you be running heat mats with these or gecko rack with heat cord? What's the bottom of the tub look like, can it run flush against a heat mat? This is the reason I would go with Sistema as they are good quality and have flat surface.