Okay, I posted this pic in another thread. But I noticed that this could be good for scaling purposes as well. Note the size of people with relation to the ships (both real world navy, and starships).
As for this pic... the 2 guys on the Nimitz are about 2x the height of the people/Jaffa in the Kheb pic. Therefore the Nimitz should be about 1/2 the size of a Ha'Tak. Which the Ha'Tak size measurement of about 600+m long to be about correct, as the Nimitz are about 320m long.