Team Torchwood are old hands at dealing with invasion attempts; there have been so many they’ve become routine. This one, however, is a little out of the ordinary.
The aliens broadcast their intentions from orbit, so the team get a good look at what they’ll be dealing with while Tosh translates their message. The fleet is landing and the people of Earth are to surrender immediately.
It’s a bit anticlimactic; the fleet does indeed land on the Plas, but that’s as far as the invading force gets because somebody steps on it.
It’s hard to judge actual size from pictures.