Listen to your body after donating. If you are light headed, take it really easy for a day or two. Some take longer.

Your physical conditioning is not a good indicator on how well you do after you donate blood.

If you are bigger and heavier, I think it helps than those who are petite. I would suggest you take it easy on your first time and just play it safe. And if you do work out have a close friend with you just in case you get dizzy or faint. Probably stay off the bike for a day or two again just to be on the safe side.

And yes I have donated on numerous occasion after I became Rubensque not proud. No amount of cycling is getting rid of the weight. Or other exercises.