If they are sweet and taste good, pick them and eat them. They won't get any bigger.

They will not last until mid-September. Most of the "table grapes" are harvested in July and August.

They are small because you need to thin the grapes, especially during the first few years of production. It's painful but you'll need to remove some of the bunches next year before they start developing. I would say no more than 5-8 bunches per vine.

As lazygardens said, pick them now because they won't get any bigger. In fact, if you keep waiting, they will turn into raisins (unless you want raisins).

Thanks so much for the great info. It looks like I will be harvesting grapes today.
Next year I will thin the bunches.