Hey benjaminmckay,
Personally, I determine the best production building by 2 factors, the space and population requirements.
Example 1: Fruit Farm
130 supplies/hour
20 squares or spaces
50 population
Efficiency = 130 divided by 20 divided by 50 = 0.13 supplies per hour per space per pop
Example 2: Blacksmith
32 supplies/hour
4 squares or space
12 population
Efficiency = 32 divided by 4 divided by 12 = 0.67 supplies per space per pop
Hence, the Blacksmith is more efficient than the fruit farm. However, this only takes into account the space occupied by the production buildings themselves, and not the houses required to house the population that is required for the production building. Perhaps I will edit my post later to include further explanation.
Also, do note that big and small (in terms of space) buildings have their pros and cons. For big production buildings, you don't have to set as many times, so it is more easy when you're doing 5 minute runs. The downside is, if you're plundered, you lose quite a fair bit. For small production buildings, it might be a hassle to keep setting them individually, but if you are plundered, you only lose a small fraction.
Cheers,
L.R.