Generally winter squash takes more time to mature when compare to summer squash. Though it is hard to tell the exact time of harvest since it depends on varieties, weather conditions, etc., normally summer squash require 60 days to reach maturity and will be ready to pick within a week after flowering.
The size of the fruit depends on the variety, soil conditions, weather etc. For summer squash it is recommended to harvest when fruits are small and tender.
For elongated varieties pick when fruits are 2 inches in diameter and 6 to 8 inches long. And for Patty Pan types pick when they are 3 to 4 inches in diameter.
There are many varieties of squash available which are mainly grouped as
Scallop or Patty Pan type
Constricted neck type
Cylindrical to club-shaped Italian marrows type
For more information on specific varieties please see the below link