A lot depends on the type of plant that you are going to use, the squashes can really roam sometimes (cukes, zuchinni, pumpkins). I have pic somewhere of mine. My garden is pretty big though.
I had 8 cuke plants, 4 pumpkin plants, and two melon plants:
They sprawled to about 10 feet out, so they packed a total of a 10'x30' area.
I like to have at least a 3'x3' for each tomato plant, peppers I get away with 2'x2'. That allows access to all sides.
Beans, if you get bush type, should do fine with 2'x2' each, I had peas, which were climbers, and had a fence made for them that was about 6' long, and they need about 1' wide.
My broccoli is crazy, so it's taking at least 3'x3' each plant. I allow a min of 2' between rows so I can get the wheelbarrow in. Onions don't need much width, I ran them about 8" apart though each.
Cabbage needs about 2'x2'.
Corn hates me, but it likes to be planted "together", so rather than like a 12' row, plan 3 rows a foot apart, 4' long.
Lettuce I just plant in a random area, mine was a 4'x4' square.