You should be pruning in late winter and you must leave some of last years wood in order to get fruit. If you prune off all of last years wood, you will get nothing. You can tell the difference between one yr and older wood by the color of the bark.

Remy: Thompson seedless are naturally small berries. What you are seeing is somewhere near the normal berry size of that cultivar. Commercial growers achieve larger berries by spraying gibberellic acid to first thin the fruit off each cluster and then to plump up the remaining berries. They also use other tricks like girdling the trunk. You could do this. It's not that hard. But it doesn't increase the flavor or sweetness and may do the opposite. Doing a better job of pruning, thinning, and other cultural tasks can increase berry size somewhat if they are limiting factors. Commercially berry size increase is used only for Thompsons destined for table grape use or other special applications like Jumbo raisins. The normal raisins are grown with small berries just like yours. So the vast majority of Thompsons are grown like yours with small berries.

I try to cut them down every year, but man do those things grow fast! And thick vines! Some are as thick as my wrist and grow so high up in the trees there's no way I can reach them. But...I'll find a place for the new ones and keep a good eye on them. Thanks. ;) V.

Pretty darn impressive and a clever way to grow it. We have some wild grapes here (muscadines) that are about twice as tall and twice as thick as that. I'd guess ~80 feet up into crowns of forest trees.