I just received my very first Trix purchase. I got the 6 car passenger set from Trix 23446.

I noticed these cars sitting next to my Walthers HO scale passenger cars. The Trix cars are significantly smaller, in both width and length.

Are the scales off? Or are European prototype rail passenger cars smaller than the US?

Prototypically, German car gauge (max. area a car can pass through) are smaller that US. UK car gauge is a little smaller that German.
The tunnels were smaller in Europe than in the US.

Also, on the model side. US models are prototypical all the way around. However the min radius is quite large as compared to Marklin. I have US models and to run then, I need a layout with minimum R3 or about 20" radius.

In Europe, to accomodate sharper radii common to most layouts (R1 is 360 mm or 14.2 in), they will model long car lengths shorter up to 1:100 scale in length but 1:87 scale all other dimensions. (houses are much smaller in Europe than in NA, hence the desire to continue wanting small radius).

Marklin/Trix do mark in their catalogs 1:87, 1:93. or something and 1:100 with sideways arrows in their icon descriptions to inform us of the short length. Lately, some new product runs have been full prototypical 1:87 length.

I have a Fleishmann ICE 2 and the cars are longer than the Marklin ICE1.

The couplers are the Märklin short coupler. They are mounted in a NEM-pocket under the car, so you can easily change them to Kadee. Kadee offers couplers for the NEM-pocket. There are a couple of sizes available, and I have found that Kadee 18 and Kadee 17 are the best match. With Kadee 18 you are sure not to run into problems with the buffers.

Kadee 18: http://www.kadee.com/htmbord/page18.htm

Please keep in mind though, that the Kadee is not a short coupler and it is not designed with the NEM-pocket in mind. It will work, but you will loose the nice look of the cars coupled close together. You can perhaps put a Kadee knockle coupler between the last car and the locomotive and then leave the Märklin short coupler between the cars themselves.

It depends on how much you wish to use your Trix cars with your American rolling stock. Even though not prototypically correct, that might be a fun opportunity.

Ya I was looking at the Roco cars, but I couldn't find a deal on the full sets with at least 6 cars. So thats why I got the Trix 6 car set. Got them for US $330 on sale.

I don't plan on using these cars with my American rolling stock, I will leave the entire train together with their stock couplers. And not replace them with the Kadees.
I like the spacing with them right now.

For the loco with this train, I just ordered the Roco 68193. The BR 10 002 with dynamic smoking. I ordered it from an ebay seller in Germany. So it will take a few weeks for delivery.

OK here's a photo of two of the them.