For image size, you need to ask yourself: what sort of images will be hosted here? Will it be very detailed, large-scale artwork pieces or scenic shots, or smaller pictures of people/cats/etc.? The higher quality & the larger the images you expect to get, the larger you'll need to increase your upload limit.

To get a good idea, I'd suggest starting off above what you expect -- if you expect 10MB images to be the norm, go to 15 or 20. Then after some activity and your gallery is filling out, start looking at averages. If you're well above average, trim it down some. If people are complaining about the upload size being too small, find out if they need to work on cropping/resizing/resampling the images better or if they're doing the best they can and the limit is simply too small.

Videos are a very varied beast. The sheer number of formats & compression rates make the entire gauntlet varied. I've seen 30 second clips that were 5MB, and I've seen videos that were well over an hour be only ~400MB or less. What you may want to look into is a converter/player, similar to Youtube, where the raw video is uploaded and converted to a much more manageable Flash video format. These may be a bit more CPU intensive, so if you plan on uploading/receiving a lot more videos, you may want to consider moving from a shared -> VPS or dedicated as resources permit. The same principle for the images applies here as well -- give a little more than what's expected, then start taking averages.

Space per member is also tricky, depends upon the primary amount of content you have, along with the number of members you have. If it's primarily images, then a lower amount will be fine.