I'm unclear about what is farm space. How do I know how much I have? Can it be increased? Is there a cap? Have searched the forum and not found an answer so apologies if this is posted somewhere else.
I've seen both Trys and Zenima have posted a link to the wiki, which is quite helpful, but I think an image makes it a bit easier to understand it all;
This image shows to overview of the tab 'Farm' (Can be accessed by pressing on your farm in city overview, or the name 'Farm' at the senate building tab)
Red: Current population. This shows how much population is 'in use' at the moment; Population is used up by building both troops, ships (each unit has it's own population cost. Swordsmen for example cost 1 population; chariots cost 4) and buildings (When constructing a building, you will see the population it uses up)
Green: Current Maximum Population. This shows the maximum population of your city. You can heighten this by leveling up your farm, researching Plow in the academy (level 22) and building thermal baths. The farm-cap is 3500 population.
Yellow: Current Available Population (Around the right top corner). This shows how much population you can still use up, either in troops or buildings. Once it reaches 0, you won't be able to build troops or buildings anymore (Some building cost 0 population!), and you will have to increase it as explained above.
I hope this is as helpful as the previous posts were