Some information about the quality levels

The answer is "it depends". Brace yourself...

For video, the bitrate used depends for a particular quality setting depends on obvious factors like the resolution of the video and the framerate (the more data there is, the more bits the encoder needs to use to reach a certain quality level). But it also depends on how "easy" it is for the encoder to do its job; if you have a video with very little motion and a fixed background (someone reading the news, for example), the encoder can generate a high-quality image without using a high bitrate. On the other hand, a video with lots of fast-moving action (like sports coverage for example) will need to use a much higher bitrate to attempt to achieve the same quality.

It's a bit simpler for audio. The bitrate used depends on the number of channels and the sample rate (and the sample size), but for a CD standard 44,100Hz stereo the Vorbis encoder uses, from memory, a target bitrate of 112kbps for its VBR encoding at OggConvert's default quality level of 3.

Quality-orientated encoding, allowing the encoder lots of leniency to decide on the best parameters, in general produces the best output, which is why OggConvert takes this approach -- it's especially important when transcoding from one lossy format to another. On the other hand, I'm very aware that it's annoying that in general you'll have no idea of the output file size when you start transcoding a video. Fixed-bitrate encoding is definitely something I'd like to add at some point.