I mistakingly added an image that was to large. Can I edit the size of the entry or do I have to drop out and re-enter?
You have to do it the "long" way.
Withdraw and re-enter. If the challenge is getting close to full, get your replacement image loaded to your gallery first to speed up the replacement time.
/A veteran is someone who, at one point in their life, wrote a blank check made payable to their country for an amount of "up to and including my life"./
(...from the generation that still uses capital letters and punctuation...)