I've got three of the "large" (3 gallon) sized critter keepers. I love 'em.
Calculating volume based on the given dimensions won't work because the sides slope.
I pick them up and carry them to the sink for water changes. When cleaning, use a soft sponge, (not the abrasive side,) since they can scratch pretty easily. I clean mine out with hot water and either a soft sponge or a paper towel.
I like the lids, since they allow lots of air flow. I cut holes into the lid edge for the heater cords, but I've read that some people don't bother.
In the winter, I found that the lid was TOO ventilated, so I covered it with aluminum foil to keep the heat in. (Or, you could use plastic wrap, if you think the foil looks tacky. LOL