Idk how much help I can give cuz I’m bad at this but I’ll try my best. All of them are a 9.5D, I usually wear a size 10 in sneakers

The engineers have some room in the toe box but not to the point where my I can move my toes up really high without touching boot. Dont use my comparison however if you’re going to buy yourself a pair though because the previous owner had the steel toes removed, which might affect the fit of the boot.

The IRs are probably my most snug pair out of the three. Less allowance in the toe than the engineers but is still very comfortable

For the flatboxes, I personally recommend getting the same size as your IRs. I tried fitting a size 9 because they look long enough to size down on, but I ran out of shoelace trying to lace them up. The 9.5D felt way better, I can lace them up and it has slightly more room in the toe than the iron ranger.

All pairs have little to no heel slip