KEY LENS SPECS
First, not all lenses that call themselves macros actually are. All true macro lenses are primes (not zooms) and have a magnification of 1:1 ("life-size") or greater.
Selecting a focal length depends on the types of subjects you want to photograph. For a studio still life, a shorter focal-length macro lens is a good choice if your subject is such that you can get extremely close. For nature macros like insects, you may need to be farther away to avoid frightening the bug, and in that case, a longer focal-length lens like a 100mm or even longer is the better choice.
Minimum focusing distance, or working distance, is another important specification when choosing a macro lens. Your lens achieves maximum magnification at the minimum focusing distance. A very short minimum focusing distance can be uncomfortable to use, and it can make for difficult lighting situations as you struggle to keep your shadow out of the frame.