Medium size- whatever that is for your plants; too small and they tear up, too big and they are TOUGH AND CHEWY. Drying out/ dropughty conditions also toughen the leaves, they are tenderest and best when the water supply to the plants has been steady and sufficient.
Timing- best season is as soon as they get to medium size. The earlier in the growing season the better. The skins thicken and toughen the longer they are in the sun, just like our old folks here in Texas. However, a handful for any dinner is OK- blanch a little longer if you are suspicious about toughness.
Best time of day? My gardening friends tell me early in the day.
Preserving the grapes when taking the leaves- Look at the bunch and the way the leaves shade it and try to take those that are not directly shading the cluster. You might want to have a major leaf-picking of any eligible leaves just after you pick your grapes.
Keep on cookin'- Ellen