If you are planning on doing scenery you may want to consider how far can you comfortably reach to do detail work. That will be how deep your shelf should be.

 

With the right bracing, you can do 24” or more. This is a bit of overkill, but in my staging area, I have a wall hung brace that supports a 14” X 25” shelf plus a 32” X 25” drop down.

 

The answer depends on your height from the belt up, how you'll bend, or stand on a stepping stool and bend, and how much and of what type of scenery items (trees, utility poles, structures) that you have near where your elbows will swing. It also depends on how you want the trackage to flow. You can go very shallow, 4" and a backdrop for tangent tracks. Or, if you want wide curves in the 40" radius range, you'll need deeper benchwork at the corners if it's an L-shaped layout system. And, it also depends on how high your benchwork is.

It is often recommended that you mock it up temporarily, either on the floor using 2cm masking tape or using temporary benchwork, adjusting for height and depth to experiment.

I think the best support is any that will greatly minimize the chances of catastrophe. Often, they can be vertical 1X2 with braces, or rather heavy metal L-brackets, but they'd have to be quite robust AND anchored heavily to the studs behind them (as you'd know). Depending on the surface materials, whether better quality 11mm plywood (1/2") or particle board, you'd need fewer or more brackets to prevent sagging between them. This could add up quickly.

 

You can construct a shelf of pretty much any width, with proper bracing. The problem is going to be reach. Figure 30" is a practical limit. As mesenteria said, if your layout is lower, it's easier to bend in and get a few more inches of reach, but that causes back strain for most of us. You could also invest in a topside creeper, which would help you lean over the layout, but while that works for making scenery, it's a PITA while trying to run trains.