You are going to get a lot of different views on this issue. I have a set of Kreitler rollers with a small diameter roller (2.25). Kreitler (USA) makes the best rollers in the world. They are very smooth and are designed to last a lifetime. Sportcrafters also makes very good rollers in a slightly larger diameter (3.25).

There is a learning curve to riding rollers. Start in the doorway and work into it slowly. Depending on which rollers you purchase, it may take a few sessions.

The diameter of the roller will vary the amount of resistance. Smaller diameter rollers offer greater resistance without the need for a headwind or other resistance device. Medium diameter rollers (3.0 - 3.5) offer some resistance. Larger diameter rollers (4.5) offer less resistance and may eventually require the addition of a resistance device.

Rollers have significantly improved my spin and balance. The smaller diameter also makes for a significant workout.