#4 turnouts have the tightest/shortest radius on the diverging track
#8 the largest radius

For passenger cars to LOOK good, use the largest # turnout you can.  This minimizes the jolting and overhang that the car will go thru when going thru the turnout. 

On the mainline you want the #8, you can use a #6 in the yard where you will be going much slower.

As for car length, it will depend on the layout.
My layout is "small" with 12in R curves.  The 85ft cars hang out over the side of the track in the curves.   And only a few cars and the train looks "odd" on my small layout.
I use the shortest cars that I can find, 40ft for freight and the "shortie" passenger cars (even if they are not prototypical), because the shorter train looks better.

There are several things happening with short cars.
- There is the "Lionel" effect.  Your eye counts the number of cars, not the length of the cars.  So a train with more cars "looks" longer.
- A shorter train looks more realistic on a small layout, vs a long one that is almost the length of the layout.
- Short cars don't look odd going around a tight turn, like a LONG car would.

I will use an EMD F-unit rather than the longer E-units, or an Alco FA/FB rather than the PA/PB for the same reason.

I did not see where you described exactly where the derailments occur, what kind of track is there (turnouts, crossings, 19"R curves, 9-3/4"R curves, etc.  Put something at the track wherever it derail, and after a while you might see a pattern.  This helps to troubleshoot the problem.

Is it the pilot truck/wheels or the first driver wheels that is derailing?

As was said by other, check that the wheels are in-guage then check your track.
I had some cars that would derail going thru a turnout.  Turns out the wheels were picking the frog and going the wrong way.  Fixed the guardrails and the problem went away.

The center of gravity (CG)/balance point of many steam engines is rear of center.  This causes them to want to pull a "wheelie," which is asking for a derailment.  I stuffed lead or tungsten putty into the smokebox to move the CG forward.

I concur w H Lee.
I would like a short vanderbilt tender.
I just ordered 4 of the MP vanderbilt tenders to replace the rectangular coal tender on some of my SP steam engines.

My guess is maybe there were not that many road that used the short vanderbilt tenders. 
I don't know the answer to that question.

ALL RIGHT !!!
Kudos for the 44 toner.
Always wanted one (or two or three) of those critters.

Mr Bach Man
Would you please make an official announcement on the Atlas forum.
You have us drooling.