Answer: Grapes can vary widely in size depending on variety, so to some extent the size will be predetermined. Healthy vines will produce the best grapes, so make sure you are pruning faithfully each winter and following any spray regime as suggested by your local county extension.
Although grapes are not terribly demanding in terms of fertility, yours might benefit from an application of compost and/or a complete granular fertilizer such as 10-10-10 each spring according to the label instructions.
Birds can really steal the bulk of your crop, so you might try protecting the grapes with garden netting.
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