When to pick cucumbers is a tricky question. They can grow really fast, especially if there is a lot of rain. I recommend checking your plants every day or two. We were away for 2 days and at the left you can see my overgrown cucumbers! On the far right in the picture is a regular size cucumber. They are still edible when they are really fat but the seeds are bigger and they are not quite as good. So watch your cucumber plants carefully so you can harvest cucumbers at the optimal size. Size here is not better!