I'm not aware of a standard for blood draws (anyone help out here?).
However, when giving blood, the standard at most hospitals is an 18g; 20g at the smallest.
Given that you are using a port rather than your veins, I'd switch to a 20g and see if that resolves the issue.
On a side note, if you are clotting this easily, make sure that your port is getting flushed well after use or you may experience issues with the port (big bummer).