square, with 4 legs (or 2 it they're large enough to give a stable surface area to work on). Height such that you can comfortably work on it without having to stretch or bend your arms in an unnatural way while sitting on a chair such that your legs make right angles at the hip and knee and your feet rest loosely on the floor.
Adjustable height legs are best but more expensive.
Make the minimum depth such that you will have at least half a meter between the edge of the desk and the viewable area of your screen and the width such that you have enough room on it yet can easily reach every part of it without having to bend over.