What Size Do You Consider to Be a Small and Large Seroma? (photo)

From what you describe, this seroma is noticeable and can be treated. If needle drainage doesn't work after a few tries, your surgeon may consider a sclerosing agent injection. If that does not work, it can be excised.

From the posted photos you have a moderate seroma collection. It needs drainage or some intervention. 

Thank you for the question and pictures.

The size of your seroma  is not concerning;  the fact that you are 6 months out of your tummy tuck surgery is concerning. At this stage, there may be a “capsule” around the  fluid collection;  surgical intervention may be necessary if aspiration attempts are unsuccessful.

I would suggest continued follow-up  with your plastic surgeon.

Best wishes.