I alternate between sitting and standing at my bench, and use an adjustable height stool as well. Drafting stools with wheels are nearly perfect, though right now I use one my wife found at Sears that doesn't have wheels.
I built my original bench to a surface height of 42". This is taller than many, but allows for easy transitions from sitting to standing and back without bending, etc. I placed it a hair lower than my elbows while standing. It is 36" deep and 96" long. Frame is 2x4s, top is 1/2 OSB. Terrible idea to use OSB, don't do it!
After moving to my new house, I decided to expand my bench into an L shape. I built a second bench that almost mirrors the first, except I used 3/4" particle board as a work surface and painted it (much better than OSB!). I then screwed the two benches together so I could eliminate the front leg in the corner.
The picture was before I mounted my press and vice, and before adding some shelves. Very sturdy, plenty of work area. I also mounted a spare TV and connected a computer to it, so I can research loads/gun/whatever without going searching for the laptop that my wife is probably already using =p. Also good for Netflix while sorting brass.