The partisan dynamics will be very different. At 6000 members TTL's equivalents of Blue Dogs and Rockefeller Republicans will be the size of full sized party blocs OTL. I think you will see stronger, proportionally smaller, ideologically purer, parties or semi-independent subparties as the threshold is much lower (25,000+1 assuming a full 50,000 person district with everyone of age and voting versus 350,000+1 in a 700,000 person district) and given the size of the legislature bargaining on a 1-1 level seems impractical.
Procedure and organization seem likely to be problematic. How you are going to handle committees, amendments, debate or even voting in a legislature the size of a small city escapes me. Even when you get to the point in time where you have TVs, microphones and voting by electronic device you still only have a finite number of hours in the day to have every person or group have their say.
I think this will have some interesting effects on the relationship between the branches as time goes on. On the one hand the legislature may seem more legitimate as the legislators are more local and in theory should have fewer people represented by legislators they voted against. On the other hand it might become slightly less powerful as to get anything done you have to get at least 3001 people to agree.
I wonder what will happen to Washington ITTL. With a legislature in the thousands and their associated staff you will have a significant difference in the population of Washington when Congress is in session to when it is out.