As for engine size comparisons...
![20201201_195612 20201201_195612](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234254940547/fodoid/152477234254940539/imageType/MEDIUM/inlineImage/true/20201201_195612.jpg)
![20201201_195618 20201201_195618](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234254940548/fodoid/152477234254940540/imageType/MEDIUM/inlineImage/true/20201201_195618.jpg)
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.
![20201201_200135 20201201_200135](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234254940549/fodoid/152477234254940541/imageType/MEDIUM/inlineImage/true/20201201_200135.jpg)
Now here is that same 261 in the middle, a 263e on the left, and a familiar face on the right.
![20201201_200226 20201201_200226](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234254940550/fodoid/152477234254940542/imageType/MEDIUM/inlineImage/true/20201201_200226.jpg)
![20201201_200206 20201201_200206](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234254940551/fodoid/152477234254940543/imageType/MEDIUM/inlineImage/true/20201201_200206.jpg)
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.
![20201201_201330 20201201_201330](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234255594154/fodoid/152477234255594142/imageType/MEDIUM/inlineImage/true/20201201_201330.jpg)
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.
![20201201_201655 20201201_201655](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234255594155/fodoid/152477234255594145/imageType/MEDIUM/inlineImage/true/20201201_201655.jpg)
On the other end of the scale is a lionel 2600 series caboose, 2657 I think.
![20201201_201548 20201201_201548](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234255594156/fodoid/152477234255594146/imageType/MEDIUM/inlineImage/true/20201201_201548.jpg)
![20201201_195649 20201201_195649](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234255594157/fodoid/152477234255594147/imageType/MEDIUM/inlineImage/true/20201201_195649.jpg)
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".
![20201201_201504 20201201_201504](https://ogrforum.ogaugerr.com/fileSendAction/fcType/0/fcOid/152477234244813798/filePointer/152477234255594158/fodoid/152477234255594148/imageType/MEDIUM/inlineImage/true/20201201_201504.jpg)
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!