One of the more practical features about building your own workbench is that you are able to customize the dimensions, depth, width, and especially, height so that it suits you perfectly. If your workbench height is off by just a few inches, it can really make things hard on your back when you are doing long tasks like sanding for example.
In general, a workbench height will be in the range of 34 inches to 38 inches. Some even build theirs up to 42 inches depending on the kind of work being done.
The reason why a workbench height is variable from person to person is that it depends on the height of the person and the majority of the types of tasks that you will be doing on the workbench.
A good rule of thumb is to make the workbench table the same height as the distance from the floor to your wrist when letting your arms hang down. So if you stand next to your table, the height would be exactly where your hands would be touching it if you put your palms on the table.
A different version of that is to us the same method but make the height at the level of where your pinky connects to your hand, which is about two inches lower than in the previous way.
There are factors that will affect this height however. For example, if you will be doing alot of detail work, probably you will be sitting for tasks like these, and so the workbench would be higher to allow for sitting on a stool.
On the other hand, if you know you will be doing a lot of hand planing or sanding tasks, you might want to lower the height to make it more comfortable for you.
Another handy tip is to remember what you will be standing on. Many people put down those foam play mats to cover a concrete floor. This will add another half inch or so that you will want to add to your table.
Ultimately, you need to make the workbench height as comfortable as you can for the work that you will be doing. If possible do some testing on your kitchen counter tops to see how comfortable you are and then take it from there.
Lastly, you might be doing a variety of tasks that sometimes would make a higher or lower table height more comfortable. Since in a garage you might not have the room for two workbenches of different heights, think about making an adjustable height workbench.