Read STL dimensions and resolve unit mismatches
Use the exported coordinate frame
Each number is the largest coordinate minus the smallest coordinate on that axis. For example, if the left edge of a plate is at X = −10 and the right edge is at X = 70, the X span is 80. Moving the whole model by ten units before export shifts both endpoints equally and leaves the span unchanged.
Check a distance you already know
Choose a part whose longest edge is parallel to one of its export axes. Compare the readout with that edge on the drawing, including the unit printed beside the drawing dimension. An 80 mm edge is about 3.15 inches. If the file reports 3.15, establish what the exporter and slicer each assumed before applying a conversion. Converting the same part twice introduces a new error.
Allow for orientation and outliers
An 80-by-60 rectangular plate rotated within the XY plane before export can have an X span greater than 80. Turning the camera afterward does not undo that export rotation. A distant fragment or protruding tab can also enlarge a span. Inspect an edge-on view, then return to CAD to measure the specific face or feature you need.