The detachable map/tablet holder is rectangular, approx 20 x 25 cm. The bag itself is approx 20 x 30 internally but, as you say, not really square so I doubt you'd get something (rigid) 20 x 30 into it.
I don't have a small laptop/big tablet so haven't tried to fit such a thing into the bag - but it's just right for camera, gloves, snacks, water bottle etc, and, with the expansion zip, will also hold those odd unexpected purchases ! (I find it very difficult to avoid coming away with another pair of gloves when I visit bike shops
)