Tiny piece - thumbnail sized (or 1cm cube size whichever is easiest to imagine). Also I have a special place for veg and not in the food bowl. It used to be in a small sputnik but now it's on top of a ceramic thing. I put it out early evening and Charlie nips out and nabs it as soon as he smells it
Fresh stuff gets eaten straight away usually, as long as it's a small enough piece. Charlie's main reason for getting out of bed is his daily piece of veg and as I found when we were away for a couple of nights, he got really quite sad when it wasn't there!
Having it in a different place to the foodbowl means they go to find it, which is fun, and it doesn't get bits of hamster food stuck all over it