What sort of work will you be doing on it?
Basic bench is pretty simple: 4x4 legs, a "belt" of 2x4 about 6" off the floor all the way around to stiffen the bottom of the legs, a belt of 2x4 all the way around flush with the top of the legs to screw the top to. On a 5' bench, a leg on each corner should be enough. If you expect to do really heavy work, another leg in the center, front and back, may be a good idea.
I use two pieces of 3/4 plywood for the top. Bottom piece screwed or nailed to the 2x4 perimeter, top piece screwed to the bottom piece.
The two piece top has two advantages: It's WAY stiffer than a single piece, and when the top gets too beat up, it can be unscrewed and flipped over for a fresh surface, or replaced if both sides have been used.
If you're REALLY gonna be beating on it, you may want a piece of sheet steel on the top. Or for more delicate work, perhaps some Formica or Masonite.
Size the frame to leave a 2" overhang on the front for clamping things to. If it will be accessible from the sides and back, leave an overhang there too.
If you plan to mount a vise, plan for that when framing it.