there are size limites, 1Km³ for parts and 50Km³ for assemblies but my system gets really slow with just a 1m³ tool assembly!
references have a lot to do with performance if all your parts are mated together and are all referencing each other that's going to majorly impact performance
I also work in the Heavy Structural Steel Industry and that is the first time I have seen Suspension Bridges modelled in SolidWorks.
That's not to say (as you have proved) that it can't be done, however in my experience nearly every job I have worked on has been modelled using Tekla X-Steel. This seems to be the industry standard for this type of work as it comes with all your connections, bolts etc as library features. As well as auto numbering and part / assembly and GA drawings creation.
If possible could you share one of the models of a bridge you have designed in SolidWorks.
I would be really interested in seeing how you do this.
Mark (SolidWorks 2016 sp3)