It's not quite so clear-cut. A narrow gauge 28' box car done to 1:22.5 can be (and often is) roughly the same size as a 40' box car in 1:29. (That's ostensibly why Aristo-Craft chose 1:29 as their scale--so their cars looked good with LGB's and the like.) In some cases, a model can actually represent a prototype car in a handful of scales. Also, when you have manufacturers like LGB who "fudge" the proportions of a model so it fits the rest of their product line, it's virtually impossible to give it any scale, suffice to say it "more or less" fits in with the rest of their product line.
It's far easier to pick a genre of railroading you like, chose an era, and then tailor your choices to match.
A thumbnail of the major players:
Aristo-Craft: Mostly 1:29 models of standard gauge prototypes. Notable exceptions: "Aristo-Craft Classics" line, 1:24; their brass speeders and railcars, of varying scales.
Bachmann: "Spectrum" series models are 1:20 (3' gauge prototypes). "Standard" or "Big Hauler" product line is 1:22.5, also of narrow gauge prototypes. This product line is size-wise compatible with LGB, Ariso, etc. The "Spectrum" product line is noticeably larger.
LGB: Scales vary based on model. The LGB "program" dictates that all models are roughly the same size so they look good in a train, regardless of prototype, era, etc. When they started out, they had a stated scale of 1:22.5, which is what prompted Bachmann to use that as their stated scale. Aristo, as stated above, chose their scale so their standard gauge prototypes would be visually compatible with LGB's trains.
USA Trains: Mostly 1:29 standard gauge prototypes. Some models (their wood refrigerator cars, wood box cars, and some others) are stated to be 1:24, though are--in reality--almost carbon copies of LGB's cars. Again, they visually play well with the rest of the product line.
Hartland: Stated to be 1:24, but--like LGB--tend to be a bit fast and loose in that regard so that they "look good" with other brands.
Accucraft: Depends on model. All offerings are accurately scaled to their stated scale, though Accucraft makes (or has made) models in 1:32, 1:29, 1:24, and 1:20.3.
AMS (division of Accucraft): 1:20.3 models of 3' narrow gauge equipment. Similar in size with Bachmann's "Spectrum" series; noticeably larger than other brands and scales.
AML (division of Accucraft): 1:29 models of standard gauge prototypes. Compatible with Aristo-Craft and USA's models of the same scale.
MTH: 1:32 models of standard gauge prototypes. The "proper" and historic scale for standard gauge trains running on 45mm track. The trains are noticeably smaller than their 1:29 cousins. Some say the difference is negligible, though it's a 10% difference in size.
Based on your moniker, avatar, etc., I'd bet that you're a fan of standard gauge trains, probably the 60s/70s era, possibly later. In that case, I'd stick with USA and Aristo-Craft's line of 1:29 trains. They both offer models that fit that era very well. Some of LGB's standard gauge models may work, though they could be a touch on the large side.
Later,
K