If you have any length of hedge, a hedge trimmer will make light work of keeping it neat and tidy. They are available either electric or petrol driven, the petrol ones are heavier but give extra flexibility as there's no mains lead to limit where you work.

The size of tooth gap on the blade will determine the size of the branches which can be cut, for example if the tooth gap is 12mm, the blade will cut branches up to 12mm diameter - it is simple, if a branch cannot get into the tooth gap, it cannot be cut. So have a look at your hedge before buying a new hedge trimmer and see the size of branches you are going to be dealing with.

Different models of hedge trimmers have different lengths of blade, this won't affect the size of branch which can be cut, just the amount which can be cut at one time. While longer blades will cut more it one go, they can be less easy to control and require a more powerful motor than a shorter blade with the same tooth gap.

A hedge trimmer is a two handed tool, there should be a handle for both of your hands.

For smaller hedges, it is probably worth using a powered hedge trimmer, a pair of garden shears will be just as quick and should give a better finish.