First of all some information about the blue ‘i’ in the ring menu. Wait, ring menu? Yes, the menu that opens when you click on anything on your farm. It holds a blue button with the ‘i’ of information on it. You can also find this button in the build menu when the building has not been built yet, but has unlocked already. Has the building not unlocked yet? Well then there are three things you can do. 1) go to a farm from a higher level player and estimate the size. 2) ask on the forums. 3) scroll down this post and find the sizes to all buildings there!
Alright, the blue button. What does it show you? Well, when you click on it, the menu like the one on the image below should be opened. On this first page you can find how large it is (red); from left to right and top to bottom (blue) the building time of the building or upgrade, the amount of XP it gives for building it, how many workers it needs, the (minimum) running costs (depends on the building whether minimum or real time influenced by happiness) and the maximum amount producible units (of the product that produces the most units in case of the bakery). Under the picture of the building (purple) you can change the level of the building to see how it improves on higher levels. The second tab of the menu shown on the second image below tells you how much input is needed to produce how much of a certain product. In the case of the mill it shows for example how much corn you need for how many units of chicken feed. Also here you can see how the producible amounts change per level.
Then finally some help for those who have not unlocked certain buildings yet or are still struggling to find how large a building is. (click on the picture to enlarge further)