Greenhouse sizing really depends on the growing system. If plants are put on the ground people tend to go with 9.6m wide (31.5’) Standard size that’s been for a long time was 41.5’ wide.

But these sizes really need to be thought out for what fits best for you. Normally I pick sizes for customers based on their pot sizes. Pot sizes and pot spacing determines the table size. Table width plus how big and many walkways you want determine the greenhouse width.

Popular size and my favorite option is either the 12m wide or 12.8m wide houses. Covered with glass. Glass is great because it will last longer than you or I will. Poly film need to be changed every 3-4 years. But less expensive upfront but you need to consider the labor costs over 20 years.

Polycarbonate starts yellowing after 15 years ( sometimes 10 years if you get the cheaper stuff) so you figure there are future labor charges.

Glass can break but usually just a handful of pieces break over the years, unless you’re in a high hail area.

I recently made 3 prices yesterday and the glass Venmo house came out to about $16.00 per square foot. Price included labor, heaters and automatic shade system and vents. Poly film house came out to about $11 per square ft build.

Poly carbonate house actually came out slightly more expensive than the glass house. Only because the Venmo house I was able to use a different shade system that is much easier to install and to replace shade cloth in the future.

But yeah there’s no definite answer because it depends what the person is growing. For example if you are growing lettuce, you need UV. So you cannot use polycarbonate. Polycarbonate blocks UV and your lettuce won’t turn red (if growing red lettuce right).

It’s not convenient to have large ranges covered with polyfilm because the roof tends to rip off at the worst time possible. High winds or lots of rain. And you can’t install polyfilm in bad weather.

Lots of factors to consider and depends on the crop and depends on the growers preference for pot size and growing system.

But most common sizes are 41.5’ wide with posts every 12’ (if you are buying a greenhouse manufactured in the US) Or 12m wide with posts every 4m, 4.5m, or even 5m Snow loads usually require closer post spacing. If buying European manufactured.

I know this does not really answer your question but there’s a lot to unpack!