As for engine size comparisons...
Above is a side by side between a usra 0-6-0 and a 261e, both mth. A postwar us army 41 and a Hornby clockwork engine in the shots as well.
Now here is that same 261 in the middle, a 263e on the left, and a familiar face on the right.
Above are some more comparison shots between the red 261 and blue 263, with a RDC car for visual reference.
Getting back to cars I can show you a few. Let's look at cabooses.
On the left is a mth 2817, in the middle is an mth steel side scale caboose, and on the right is the lionel traditional size Halloween caboose.
On the other end of the scale is a lionel 2600 series caboose, 2657 I think.
Here are some marx tinplate on the near track, with 2600 series on the center track behind the 261. The marx cars are considered 3/16" size as opposed to 1/4".
Here we have 2600 series gondola on the left and a 2800 series car on the right. The green gondola has been modified by me to have a knuckle coupler on one end. I highly recommend that everyone builds one of these, it makes running mixed styles together possible and is a fun, easy build.
This is all I can do for you right now. I highly recommend the 263e you mentioned initially. You could use 2600 series passenger cars or 710 series cars. A far warning, 710 series is cool looking but can be hard to come by.
I hope this helps you add. As a second warning, tinplate is addictive- take a walk on the shiny side!