iIf they're dark green with a slightly prickly skin then, as nettyann says, anything above 6 inches up to maybe 9 inches. Above that the seeds tend to get a little tougher.
If you can find the seeds or plants next year try Armenian cucumbers for a real treat. They grow up to 18 inches long but still stay about 2 to 3 inches in diameter. Very tender and one doesn't need to remove the skin.
They grow best if staked up since the fruits grow so fast they tend to curl as they grow... 2 plants or two hills of seeds are plenty for most families...