You're not limited to 99 pictures/songs. You're limited to 99 chapter points.

What this means is that for the first 99 items, you can click the >>| and |<< buttons on your DVD player remote to jump between items. Past the 99th one, you would have to use the fast forward and rewind button to navigate.

Is this 99 in each?

Yes. If you split your pictures/songs among multiple compilations on the same disc, each one can have 99 chapters. Again, you can have more items; only the chapter points are limited.

Can you get beyond the 99 limit if they are accessed from a seperate menu?

Actually, you can put several compilations on a single menu, and each one can have up to 99 chapters.

If you have a music compilation how does the AC3 stereo file size compare with that of say an MP3 file?

That depends on the settings used to encode the AC3 or MP3 file.

At any rate, DVDA won't create a disc with MP3 audio, so you haven't really got a choice. If you import an MP3 into DVDA, it will convert it to AC3 (or PCM, depending on your DVDA project settings).

Rob