A grip must be comfortable; otherwise you will feel fatigue at the end of the fishing day. You will also get a wrong impression about the action. Too thick a grip is better than a too slim one.
The dimensions must be in harmony with the rods length and intended purpose.
It shall be as long as needed, not more, or the handle will "steal" action from the rod.

Recommended lengths:
7 ft, 13 cm
7 ½ ft, 14 (15) cm
8 ft, 15-16 cm
8 ½ ft, 17 (18) cm 

The picture shows three rods:

A typical "Midge grip", where your hand will cover down on the reel seat. Not good but better than lengthening the handle out of proportions.

The "Payne grip "is an 8-footer for # 5-6. This type is associated with fine classic rods like Payne's, Gillum's and the most popular today. If the depression behind the centre is more pronounced it´s a "Western grip ". Both are comfortable in the hand and the first choice for many.

The "Wells grip "is not seen very often these days. It´s a pity for it is excellent where thumb thrust (for long casts) is needed. It was the standard grip on most European rods in the past and should be considered for rods of 8 ft and up.

Fishing a big river always calls for thumb thrust.