Depends on whether you're talking about whole blood or PRBC's (or something else). A blood donor gives usually between 450-500 mL of whole blood at the collection site, depending on the site's SOP, the donor's size, and the speed of the collection (may be a little light if the flow is too slow or stops). In the lab, they usually separate the components and you end up with 300-350 mL of red cells usually, as the other posters stated. Hardly anyone gives whole blood anymore.
Actually it is not standard for any category of cells. It varies according to whether the donation may have been cut short due to problems, and due to the amount of excess fluid removed. And since, we rarely give whole blood (except in "the field" where measurement would be difficult, it depends on amounts removed due to leukoreduction, and removal of plasma for FFP. In addition, though plasma pheresis is preferred for transfusion, there are still places that use random donor, which is removed from the RBCs.
An average PRBC unit is 275-475mLs. Some hospitals, have volume depleted units for use with patients that have fluid status issues...more of the excess fluid has been removed. Also, patients that require blood run very slowly, and the units are divided.
Platelets - one unit pheresis (single donor) - anywhere from 175mL - 500mL. If 500mL, then it usually is in two bags. Platelets from random donors vary even more widely.
Don't they add additional fluid in the lab sometimes for preservation purposes? Someone once told me that but I don't know for sure. I think they also add a perservative chemical.
You are correct. The most commonly used anticoagulant/preservative is CPDA-1, which contains citrate, phosphate, dextrose, and adenine. This extends the shelf-life of a unit of PRBCs to approx. 120 days.