It's not exactly what you're looking for but I have this old size comparison that should give you some idea of the difference in size between
40k and Legion models.
In addition to being a bit larger than
GW models in general, Legion models tend to have oversized heads. You should expect the Fleet Trooper heads to be about 25% larger than Guardsmen heads. They might not look too out of place on old Cadian bodies since they were pretty bulky models themselves, but probably stand out and look ungainly on modern
GW plastic humans. But I haven't assembled a new Cadian or Krieg model myself yet, so I can't say for sure.