Something to keep in mind is that you may have more than one use, for example print and web. In that case you always need to shoot for the largest size and rez down (decrease image size). If you have ever seen a horrible, jaggy reproduction, you are seeing something that was shot for the web which someone tried to "rez up" for print usage. It doesn't work.

What you need to ask yourself is if you want a big file "just in case" or do you have a specfic purpose for the larger file right away. Because the larger the file, the more time and handling is involved. A 30-50 meg file will show a lot more detail that needs to be retouched than a 350k thumbnail file, it will entail more time and handling, and more cost.

Also, do you need CMYK conversions for prepress, or does your designer want RGB files? More about this in the "Who Is Involved" tip.