I always add oak during primary fermentation, 2-4 oz is a decent start, you can always add more. I add chips in a nylon drawstring bag, this way I can remove all at once. Taste it often, twice a day to make sure that it isn't over powering. Oak will dissapate as the wine ages.
Another way to add body is to add raisins to the primary, I rehydrate in a few cups of water first, I'll use 2-3 cups in 5-6 gallons of wine, this adds nice mouth feel, body, and to some extent, acts as a yeast nutrient.
I'll chop them, rehydrate and add to a drawstring bag, they are a PITA to remove.
Tom