I've been asked to supply an .avi file for a Web site. Is there an optimal size I should shoot for? Total video is 11 minutes. Obviously larger is better, but then I suppose it may have trouble loading for some people. Is there a rule of thumb for this? Also, the reason the Web designer wants an .avi file (I'm on a Mac) is so that it will play in flash. Is that the best codec to use, or should I suggest another one?

This person has no idea what they are talking about. Ask them to send you a working sample of an AVI file that works. Then note the bitrate and codec used, and try to provide the same thing. You may not be able to from the Mac.

I don't think she has an example to send me.

She could be requesting the AVI so that she can do the encoding in Flash. The full Adobe flash app includes an integrated video encoder.

But, if not, as Perrone notes, AVI is about the last choice for actual online distribution. OK, it's not really a choice, period.

If she's doing the encode, for standard definition or lower in the web page, just an AVI-DV is fine. If high definition, you'll need to work out what codec. If you have the Cineform encoder she could get the decoder by loading Cineform player. Or, Lagarith, or AVID DNxHD.

If you're doing the encode for distribution there are a lot of choices. It would start with how big she wants the video to be in her design. The eventual delivery would be as an flv or swf to integrate with flash, created with the VP6 or h.264 codec.

Not all h.264 encodes are playable with flash...(more!)