PCT TRACON Controller requested changes to Class B airspace to enhance safety.

Narrative

I was just on departures and watched everything. Aircraft X was descending towards the initial approach fix during simultaneous ILS approach operations at IAD. The final was busy as was the approach sector who handed him off missing many VFR targets not talking to flying all through our arrivals and the final. One VFR [aircraft] flying west to east level at 3;000 feet flew right through the final just outside the Bravo and Aircraft X who was descending to the same altitude responded to a TCAS RA to miss him. The VFR target was hard to see with all the busy traffic for the west final over top of his tag; and the final was busy. This happens all the time.We need to extend the Bravo to encompass our finals. FDK airport is just under about a 25 mile final to IAD when we are in a south operation. Our simulate SOP dictates we must go out to just south of FDK but usually get pushed out farther at 3;000 feet. This means aircraft fly into the Bravo; past the airport; then back out again; not just out of the lower limits; but out of the lateral confines of the bravo all together. Furthermore; the SFRA causes almost every single GA aircraft on the East Coast that wants to fly into the Northeast corridor (PHL; NYC; BOS area and beyond) and avoid having to talk to ATC or file and SFRA flight plan to all fly around it over the same spot. This is right over FDK. There are far too many little VFR [aircraft] that we aren't talking to that we are constantly trying to dodge flying right through our finals and arrivals. Even when we are north; we have two major STARs that fly right over FDK. The Bravo needs to extend out to encompass the FDK area to push these aircraft out of the way. The same problem exists but to a lesser extent in a north operation with HEF aircraft.