veign,
depending on the computational resources, here's what might be simpler (which is always better!):
just one checkbox, saying:
[] creat versions of this at varying sizes
and then 2 edit boxes for resizing to a specific size.
the "Varying sizes" option should produce a page of resizes of varying sizes, i would suggest something like
10%, 25%, 50%, 75%,
16x16, 32x32, 64x, 160x,240x,320x,480x,640x
The idea being that this will be like a portfolio page where they can visually see what the image looks like in different sizes and choose which they like, rather than having to guess ahead of time what they want.