Hey T-Bob, just a little expansion on what Steve said,,
When you pick your shocks there are many considerations. In the area of length here are a couple of things to remember. The shock needs to be able to travel (or said another way the suspension needs to be able to travel both directions) without the shock hitting bottom or the other way without the shock hitting full extension.
So you will need to know those numbers to be able to select a shock the right length. If you have a total length of suspension travel of say 8 inches, then you would want a shock that has a bit more travel than that, at least 8 1/2 inches or more. this would give you 1/4 of an inch each way to keep the shock from getting damaged by hitting the end of its travel.
Remember shocks are not ideally meant to support weight, or limit travel. They control the movement of your suspension during travel, that is the ideal situation. Of course there are air shocks and others, but that is a different disscussion.