This site seemed rather useful, so I figured why not post it:
RitchieSpecs has specs on many (most? almost all?) models of modern (and not so modern) construction and agricultural equipment - for example, here’s the spec sheet on the Cat 315C Hydraulic Excavator, which in available in HO from Norscott - from a quick check w/ the specs and a scale rule, the model width to outside of tracks is about a foot less than the prototype, while the length of track “on ground” is about half a foot short. Some, but by no means all, of the specs include prototype images - and keep in mind the schematic drawings they use for dimensions are generic representations of the model, so don’t panic if the design doesn’t match your model exactly - the dimensions are what count… Most of the modeling world’s favorites, such as Cat, Volvo, Liebherr, Kalmar, JCB, John Deere et. al. are represented.
I’m mostly interested in things like max loading height, to get an idea, for example, of whether an excavator or wheel loader model would look reasonable (or conversely, ridicolous) posed next to the gondola it’s supposed to be loading. Also nice to see if the model is underscale, overscale, or pretty much spot on in key dimensions.