The 1799 halfpenny was 30.5mm and 12.5g. Given the wear on your coin, 11g seems reasonable.
Also, the legend is clear enough to see that the "S" in "GEORGIUS" is roughly level with the back centre of the head, in the 9 o'clock position. In the 1806/7 issues the "S" was much higher up the head, at about 11 o'clock.
I agree with bd251.