Do you mean the actual size as it appears on screen or the file size which is perhaps too big to upload?

If it is just the former then in the description tab just click on the pic to hi-light it and then drag the corners in to re-size it.

If the file size is too large you will have to use PB or an image editor to reduce the file size, most cameras come with their own software or there are plenty of editing tools available.

I actually have my camera set at the correct resolution for uploading to ebay.