A good rule of thumb in pruning grapes is to leave approximately 40 dormant buds on the fruiting canes when you do your winter pruning. If your vine has 4 main "arms", that means cutting them back to 10 buds per arm. At the same time select another 4 shoots to cut back to 2 buds which will sprout and just make leafy growth this year, but will serve as the fruiting canes next year. Fruit size should be full size from the outset. If fruits are small, either they are under stress or too many fruiting buds were left on the vine. It's almost impossible to prune the vines too much.
Did the birds get under your netting last year, or did they reach through it to get to the grapes. You may need to suspend the netting on posts above the vines so that the birds can't reach the fruit. You may also need to firmly attach it at the base to prevent the birds from getting in underneath it.