Hello and welcome to FF! Most full grown Pacman's will do fine in a 10G tank. Maybe a female Ornate or Aurita will require a 15G tank, but that's about it. Your 20L would be a nice home for couple toads, Tomato Frogs, or similar.

Going back to Pacmans, as previously mentioned, a baby will be fine in a smaller enclosure. I use ExoTerra's Medium Breeding Boxes inside a 10G. You can cover the whole 10G top screen in aluminum foil; then lay light dome on top in the intended position and trace outline with a sharpie. Finally use scissors to cut along line; but make the hole 1/2 in. larger. That will help keep heat and humidity in. A 60W Ceramic Heat Emitter should do; but you can go larger if dome has a dimmer like the Fluker units. Heat pads are very inefficient when used on the side as required with frogs; so if going that way, get the largest you can apply.

Do get a combo digital gauge; even if using a Hygrotherm or other controller. Need 82F and 70-80% humidity for a baby (can go to 84F for adult). Night drop can be adjusted in Hygrotherm or manually with dimmer if no Hygrotherm is used. You only need a few degrees drop (78-79F is fine).

Use treated tap (Seachem Prime) for all water needs to include mixing the shredded coco substrate. Can mist with distilled water; but avoid spraying frog.

Best food are not dyed Night Crawlers (from Walmart or local bait shop). Can cut those to mouth size for babies from pointy end. Couple in. frogs can handle pieces same as body length and 3 in. frogs can handle whole worms. Crickets should be sized same as distance between frog's eyes and gut loaded with carrots, lettuce, and crushed oats/cherios. Good luck !