Sunshine Skyway Bridge = Length 29,040 feet
(I think it is the 2nd largest bridge in the USA)
The largest one in the USA would be Chesapeake Bay Bridge-Tunnel, Cape Charles, Virginia = total length = 89,760 feet
You have to wonder, though, if target selection will be a last-minute kind of thing. I figure they're casing lots of locations, and ruling out those that carry a high risk of getting caught, or a low probability of success in killing people or causing structural damage.
It's not nearly as hard, in my estimation, to carry out a truck bomb attack, once one has the explosives, as it is to pull off the hijackings. If these guys have Ryder trucks loaded with explosives and ready to go, it's just a matter of 'can they get the trucks to the target without being stopped and searched'. They might have a decoy truck for each real truck. The driver of the decoy truck would be obviously Middle Eastern (perhaps complete with turban and beard), and would make a grand show of acting suspicious, furtively darting his eyes around. He'd park the truck somewhere ominous and run like hell. If apprehended, he'd swallow a cyanide pill. Meanwhile, as the authorities descend on this decoy truck, the driver of the 'real' truck quietly places it at the target and pushes a button (or lever, or however they set these things off). He'd stay in the driver's seat, ready to enter paradise.
You've gotta figure the FBI has guys sitting in a room coming up with all sorts of scenarios like this. I would not be surprised in the least if they've called Tom Clancy to see what he thinks is a possible attack scenario.
There have to be more qualifiers on that record. Maybe you mean the 2nd longest suspension bridge. The bridge over Lake Pontchatrain near New Ortleans is 26 miles long. I don't even think that is the longest.