I just tried it. The simplest command I tried first was:
ffmpeg -i video.ogv out.flv
The flv file came out MUCH bigger than the original ogv file. So I tried to shrink the frame size:
ffmpeg -i video.ogv -s 400x300 out.flv
The file came out smaller this time, but you couldn't read anything from the screen
I suggest you read the manual page ('man ffmpeg') too see the command-line usage of ffmpeg and then play around and tweak the parameters until you get the result you're happy with and can put it on you site.