When you import a graphic by reference, you set the resolution in DPI. If the resolution you set is the same as the resolution of the original graphic, Graphics > Object properties will then inform you that the graphic is imported at 100% of its original size.
The resolution setting in the Imported graphic scaling dialog doesn't seem to be sticky, which is a shame; that is, FM doesn't seem to remember your choice from last time.
an hour or so later … I've been working on a real document, and now it seems the resolution setting is sticky. That would, of course, make things a little easier.