Gonna add something on :]
6. Extra information about pot sizes
Generally with pot sizes you need to consider how big of a plant you want to get. Generally a bigger pot will supply much more space for roots to grow, allowing the plant to have a sturdy foundation that's capable of providing enough nutrients to support it's size. The more nutrients it takes up and uses, the better. Generally bigger pots means bigger plants.
Bearing that in mind, it is possible to get big buds and a fairly big plant from a small container, such as a 3 litre container. Generally 3 litres suits most people fine if they're doing methods such as Screen of Green (making a lot of little plants as opposed to less but larger ones)
A pot size that is considered big is roughly 15-20 litres. 25-30 litres is a bigger pot size, which means that the plant has more room, but bear in mind that a bigger pot means more effort.
By more effort I mean the amount of water needed to flush and water your plant. Generally with flushing, you run 3x the volume of the container through, or by using JM's method you use 1x pot volume of pH 7 water, then 1x pot volume of pH 6.8 water, then 1x pot volume of pH 6.5 (only applicable for soil growing, other setups require different pH's).
You also have to find the right amount of water to water your plant with. I generally use 3 litres of water in a 20 litre pot by trial and error, but you want to aim to get 10% of the water you put in running off out of the bottom.
My personal recommendations are:
For a stealth grow with small space: 5-10 litres (depending on if you want multiple plants or not)
For growing 1 single big plant: 20-30 litres
For growing multiple big plants (i.e.: 3): 12-16 litres
For growing SoG method: 2-4 litres
Generally, just use whatever space you've got for however many plants you want to grow that will fit in the space :] Most pots you can order online will have the dimensions in the item description so you can see beforehand how big it will be.