artist of the year 2012 i have entered online but i put the wrong dimensions in. the page tells me i cannot change my entry but i can withdraw it to amend it. but it doesn't tell me how to withdraw the entry

I made a mistake once when entering a competition and like you I couldnt delete it. I sent an email explaining what happened and found out deleting the entry cannot be done by you it has to be done by the 'experts' because it has been entered into a competition.
Go to 'HOME' and scroll down to 'Contact Us' and you will be able to send an email describing your problem and it will be sorted out for you.
Hope this helps.
Emmy