Judges will be looking for straight fruits of uniform thickness.

Cut the fruits off the parent late in the evening when they are fully charged with water. 
A pointed cucumber is not ripe.
Cucumbers are usually shown in pairs between 30 to 37cm long.
Cut the fruit with a length of stem; it can be handled by the stem and the bloom will not get damaged. 
The flower should be on the end, the fruit should be young, crisp and tender.
If your fruits are at their best a few days before the show, cut them off the vine and place them stem downwards in a bucket of water, change the water every day.