I suck at reading dimensions...the first # is length and then width, correct? Bout to pick one up for my 20L.
For the UGF circulation pipes?
I am not sure that it matters really but I think that the 24"x8" would be the model that you would want on a 20L (which is 30" x 12" IIRC). Since it isn't a full tank style plate like a regular UGF I don't imagine that it really matters if the dimensions go all the way to the edge of the tank.
Just remember that people are using a layer of ceramic/sintered glass bio-media both for increased biological area and to help insulate the UGF from the soil in the substrate. Many people use Matrix since it has a decent shape and quality and is available relatively cheaply in decent volumes.