When do you know to increase oz for an infant? And what is avg bottle size at 6wo?

Varies. Babies know when they are full and will generally stop eating at that time. It is easier to over feed a bottle fed baby so make sure you are watching closely for signs of fullness such as turning away, acting less interested, etc. The amnt will vary and depends on how often your baby eats during the day. Many six wk olds will take 3 to 5 ounces during a feeding.
Increased frequency. I agree with dr webb that the volume varies for each baby. When you find a volume that works for your baby to keep him satisfied for 3-4 hours and then he begins to signal for feeds sooner and sooner, it's probably a good time to try 1/2 to 1 oz more at a feed.