1/2 to 1 cup per day but no more than 1 cup.
Not much, remember they are only small. Lettuice can cause guinea pigs to get an upset stomache so try not to feed them too much of that, however they love carrot tops, etc. I feed mine a little (about a handful) every other day or else they get fat and have poorly tummies.
Hope this helps!
Around 3 vegie's
about your finger of cucumber carrot etc ..
A handfull of lettace
about 4 mouth sized (ur mouth) pieces and a big leaf of some kind - cos lettuce is the best. And lotsa water.