standard plywood sheet that you'd use for the top is 4'x8' so try to keep it within those limits to be able to use ONE slab for the top
your arm's reach is 2' to 2.5' so make that the depth - heck, maybe even 3' deep if you have the space for it
2x4 lumber makes great supports / rails, and also legs (or you can go 4x4 for legs)
google images can likely give you a TON of various plans, but most are going to be 2'x4' or so. i'd go 3'x6' for adequate space. you don't need a specific design - a ladder frame for the frame, 4x4 for legs, ply for top, nail gun, 1/4" ply sheets for side & rear panels to keep it from skewing