Call the Venue or Promoter. As soon as you know you will need to cancel the gig, call the venue and/or the promoter, contract or not. Obviously, this is important information for them to have, but this is also your chance to mitigate the fallout from the cancellation.
Instead, you change the size when you are sharing the photo. For example, if you choose to email the photo, the Mail program will ask you to choose the size of the image (I believe it is small, medium, large, original).
To view the size of a single email. Open the email you wish to view the size for by double clicking on it so that it opens in a separate window. Then click File on the top left corner, and then click Info. You will see a button titled Properties on the right and the heading Size under it. To view the email sizes of all the emails.