Frankly, if you just want to practice mixing, I’d get a set of small (10″ or so) PA speakers. This is what you will hear when you play out also and you can take these to small house parties and such, something not possible with monitors.
Monitors and their sound qualities become important if you start producing your own tracks and you need as close to “true” sound as you can get for mixing and mastering. For “regular” DJ-ing the slightly colored sound of PA systems can actually be easier to listen to for longer periods of time and usually they will pack more of a punch.
A 12″ in a small space will most likely “boom”. They are made to provide sufficient bass sound to a slightly larger room. In a small room the bass will reflect of the walls and intensify causing the boomy effect.