Nominal Hoop spacing is 4 feet O.C. on typical greenhouse/hoop-house construction , however you can space them closer if you like. On some applications such as summer shade or bird net structure applications 6 ft is acceptable provided the covering is removed before the winter season.

Typically greenhouse lengths are based on being divisible by 4 (feet spacing) For instance if you planned to build a 42 long greenhouse, consider going to 44 or even 48  feet. The longest greenhouse are normally 96 feet long this is because the poly typically comes in 100 ft. rolls and your poly must be longer than the house.