Hi Xalor, glad to hear PixelCNC is working out for you, thanks for your support :)
The reason that PixelCNC auto-fits models to the canvas is due to models being in a variety of scales that are not connected to any kind of real-world units a lot of the time - like you said yourself there can be a lot of files with weird dimensions. Sometimes they are actually modeled or generated to fit real-world units like inches/feet/millimeters/centimeters/meters, etc.. but there's no way to reliably map the randomly scaled models to real-world units because real-world dimensions aren't stored into STL models, just vertex XYZ positions. The models that aren't already scaled to any real-world measurement unit are forever stuck in their random scaling that their creator produced them in and need to be manually scaled for any cutting or printing projects.
It's possible to add an option to PixelCNC for users to import models with a scaling factor as an alternative to auto-fitting to the canvas. i.e. if a model is in centimeters when loading it into a metric project you could use a "10" scaling factor to scale it to PixelCNC's millimeter scaling. If a model is already in millimeters you would just use a scaling factor of "1", as well as for a model that's in inches if your project is in inches. Or, a model that's in feet loaded into a project that's in inches would use a scaling of "12", etc... but in my experience there are just a lot of models out there that aren't in any meaningful scale and the only thing you can do is manually scale them anyway. When loading a model using a scaling factor option you will then need to use the "Fit Canvas to Layers" function from the project's Canvas Properties dialog.
If that feature sounds useful to you I should be able to get it in there for the next update that will be released sometime in June. Let me know what you think, and if you haven't already you might want to subscribe to the Deftware YouTube channel to catch new PixelCNC tutorials and project walkthroughs as they are released: https://www.youtube.com/channel/UCMG4-1aHezxoFKaAa4_vu-w There's a lot of functionality in PixelCNC that most users are completely unaware of unless they've been following the development log over the years and it will all be demonstrated and explained over the course of a series of videos soon.
Feel free to share a project or two on the forum! :)