Technically, you can harvest them and use them at any size up to a point....with that point being the one where they get big and very seedy. With White Wonder, the cukes are usually at the perfect degree of maturity when they're about 6-8" long and 2-2.5" wide. The white will have a yellowish tint. They might not get that big without adequate irrigation or rainfall. If you cut into one and it is very, very seedy, you harvested too late. That doesn't mean it isn't usable, just that it is past its peak.