It depends on how much room do you have, and how much of that space you want to occupy.
Just start with a 4x4 tent or a 3x3. I chose a 4x4 as it's small, but still big enough to yield for myself decently.
2'x2' or 2'x4' are the smallest ones you can get. You can do 1 plant in the 2'x2' comfortably and shove 3 plants in a 2'x4'. You'll probably want to do topping and/or low stress training to keep the plants small and canopy even.