I am building a work bench in my garage. approx 16' long.

How high should it be?

Andy

entirely depends what you want to do with it. I use work tables hardly 24"high and with some jobs my back kills me bending over but 90% of what I do suits a low bench height so I go with it....

if you have any saws planers etc match those heights
would also suggest 2 heights around 150mm difference giving you more flexabillity as work pieces can range from a few mm to several hundred in height

In an ideal world, the top of a metalwork vice (you will have one won't you?) should be level with your elbow. Lower is acceptable but higher is not. You'll go mad trying to file metal above elbow height.

So choose your metalwork vice first and build the bench to suit. A woodwork vice generally fits flush with the bench top and will therefore be lower but this is no bad thing either.

Ideally just below your own elbow level, then you can lean slightly into any work.