Strannik said:

the cells are usually rated at 100% output so if you run it at 50% the life will be longer

While totally true, I don't feel this captures what is really happening. Cell life time can be thought of as producing some fixed total amount of chlorine over the life of the cell. If you buy a cell that is twice as large, it will produce twice as much chlorine in it's lifetime. Since you will actually adjust it to produce chlorine at the same rate, the pools needs some constant amount of chlorine/day regardless of cell size, the twice as large cell will last twice as long.

Yes, there is a minimum flow rate. It varies by brand/model. Typical flow requirements are between 15 and 40 GPM, depending on brand/model.