Long is generally used for a horizontal measure (something that is parallel to the ground or other surface); it is how far away one side or end of an object is from the other side or end.
Tall is a vertical measure; it generally means how far away the bottom of an object is from the top of that object.
High can be used as an informal synonym for tall, but in more technical usage, it is a measure of how far away from a surface the bottom of an object is, rather than how far away the top is from the bottom: A man who is 8 feet tall is larger than any other human currently known, but a man who is 8 feet high is probably just an ordinary man who is standing on top of a ladder (so his feet are 8 feet away from the ground).
If you want to know what is the longest, the tallest, and the highest bridge in the world, you can certainly use all three words, but you will get a different answer for each:
- the longest bridge in the world is the Danyang-Kushan Grand bridge, which is 164 kilometers from the start of the bridge to the end (that is, the distance you will travel without ever being approximately at ground level, or having the road surface be built directly on the ground).
- the tallest bridge in the world is the Millau Viaduct in France, measuring 343 meters from the base of its lowest pylon to the top of its highest pylon.
- the highest bridge in the world is the Sidu River bridge, whose deck (the surface upon which people travel) is 496 meters above the lowest point in the gorge that it traverses.
(all bridge information taken from wikipedia searches.)