dripleg size

General rule of thumb for drip legs is as follows:

Drip leg should be same diameter as the pipe.

Distance from bottom of pipe to the top of the trap (drip leg length) should be at least 18". More may be needed depending on the type of steam trap. If using a thermostatic steam trap (which requires condensate to cool before the trap is open), then the drip leg may need to be longer.

You can go to the following links for typical hook-up pictures:

http://www.spiraxsarco.com/applications/examples/steam-trapping/steam-tracing-and-manifolds/manifolds-on-critical-tracing.asp

That would depend on the steam pressure and the heat transfer rate into the process piping you are trying to keep hot. The most important thing is the size of the steam trap to put on the drip legs.

Is a drip leg the same as a tailpipe?

No actually the drip leg is perpendicular to the steam flow and is used to remove the condensate build-up in the steam tracing as the heat transfer takes place so that a steady flow of steam occupys the tracing line. The "tailpipe" in a steam system is the smokestack on the boiler.