Harvest cucumbers as soon as they reach mature size; cucumbers left on the vine past maturity will suppress the production of new flowers and fruit.
Slicing cucumbers are best picked when they are 6 to 8 inches long.
Pickling cucumbers–sweet or dill–are best picked when 1 to 6 inches long. Pick pickling cucumbers every two days.
Regular dills are best picked when 3 to 4 inches long.
Burpless cucumbers should be picked at about 10 inches long.
Hothouse-grown English or Armenian cucumbers are best picked when 12 to 15 inches long.
For more information on growing Cucumbers, check out this great guide from Harvest To Table: