While I haven't fully made the decision to build the layout here (although I'm leaning heavily in that direction again...and might move a door to make some more room), I have been to a few ops sessions.

The layouts are of a variety of sizes and in various states of completion. What I've learned so far is that the size of the layout is of some relevance, but the sessions all tend to last about the same amount of time. The real key is understanding the limitations of the layout: How many people can run trains at once, and how many people are needed to run all of the trains? These two questions are slightly different.

How Many People Can Run Trains At Once?
This is affected by several factors. The amount of space for operators, as well as the number of throttles are the two largest factors. This is usually not too big an issue, simply because the track plan also limits the number of trains that can be moving at once, and a smaller layout typically has fewer trains at a time.

How Many People Are Needed to Run All Of The Trains?
In general, some operators will run more than one train, while others will only operate a single train in a session. In most of the sessions I have attended, the local runs for pretty much the entire session. Through freights (which may have some work)and passenger trains have shorter runs, and a given crew will probably run several. The number of trains to run needs to be high enough to keep everybody occupied most of the time.

So what's different on a larger layout? Well, it depends on what "larger" means. One two of the larger layouts I've run, the track circles back in the same space. There are a number of stops along the way, and some hidden track as well. The track plan in both cases include a decent-sized yard, and the work there keeps one operator well occupied for the session.

One of these layouts is set up in a block system, and you run every train through your block, and pass it on to the next block to continue the run. The other one I have only run the local, but you basically follow your train around the layout. Both of these layouts had a similar "feel" to the smaller layouts I've run, just a little more space between the stops. The train size wasn't necessarily bigger.

The only layout that felt significantly different was significantly larger. Five people were assigned to manage the yard work. Once again, the local(s) seemed to take the bulk of the session to run. Through freights were limited to two stops, and went fairly quickly, but there were a lot of them. The major difference, though, was in the scale. Passing tracks were long, and what is a quick runaround move on a smaller layout takes some time. There is a lot more time for railfanning as you follow your train between towns, or stops if you are only making a few stops. In addition, the number of trains and the number of operators is significantly higher since there is a lot more space. This felt much, much different because of the difference in scale.

Among other things that I have learned:
-"Long" trains don't have to be that long. I believe the largest train I saw was 46 cars. Even on a large layout, that looked huge. It was also about as large a train that the yard could handle on one track.
-Short, unblocked trains can take as long to switch as a long blocked train. On smaller layouts you can extend the operations by sending out unblocked or partially-blocked trains.
-Facing point switching also extends the operations. If you are planning on a small layout, an urban area with a mix of trailing and facing point switching may be more interesting. On larger layouts mostly trailing point (or operating the locals as turns, where they switch only trailing points) can help from making the session too long. Even if you are freelancing, pay attention to prototype track plans. At the very least you need a long enough siding to drop cars and runaround the train. But you may need at least one additional siding to facilitate splitting the train if needed without fouling the main.
-Two-way sidings can also extend operations. This means that there is a stub-ended siding in both directions off of one turnout. This gives you both a trailing and facing point turnout, but can also make switching moves more interesting since cars at the end of one of the sidings may limit how many cars you can move at once on the other.
-It's OK to make people wait. You want to have some work for just about everybody early on, but during the session it's OK for them to have to wait for the yardmaster to build a train. This gives the operators time to talk, railfan, and admire the layout. Just try to keep it so that no one person is hanging around too much.

Another tip is that it's a good idea to have somebody familiar with the layout responsible for the yard and the local. I'm still a big fan of two-person teams, and you can have one novice and one experienced operator on a team.

So, while I'm still in planning mode, operations has become the biggest consideration. That may affect which portion of New Britain I model if I have to reduce the size. I'd like to keep the operations to a minimum of four people. The two local 44-tonners will occupy two, and one person or team could be managing most of the other trains. I would manage staging, which most likely will include the Berlin line trains to local businesses and the Whiting Street yard. If I'm lucky, I can move another door which will give me a location to build staging that will not be under the other decks. I'll have to see how the geometry will work...