The size of the seroma is really not that important. The fact that it is being drained is important. Removal of the serous fluid is necessary to allow for the skin flap to re-adhere to the underlying muscle/fascia. Otherwise you may develop a chronic seroma cavity that would require...
It really depends on how extensive your tummy tuck was to determine if you will be able to return to work after a week. You will most likely still have drains in so you may find some minor irritation from them. I would recommend asking your plastic surgeon what he thinks would be the best option...
Sorry to hear that you are having this type of pain after what appears otherwise to be a great result following surgery. What you are experiencing is normal. Ask your plastic surgeon if it is OK to try a topical pain cream (such as Capsacin) or if a different pain medication could be perscribed....