Where are the extrude and scale values? To be specific, if I extrude by pressing E and I type 40 (for 40mm), where do I look up this value on Blender?

Same for when I scale; I'd press S and type 0.2 (for 20%).

In a situation where I would go back to see the size of a face, how do I know if it was scaled 25% or 20%, or how would I know if a face was extruded to 40(mm) and not 55(mm)?

I need to know because I want precision or accuracy for 3d Printing.
There isn't explicit construction history in Blender per se.  If you ever want to know the dimensions of your object for 3d printing, you can always go to the display properties in the viewport, and scroll down until you see the Edge Info buttons under Mesh Display.  Note that you must have fully applied the extrusion operator to see these dimensions.


Move your cursor into the 3d viewport
Press "N" to bring up the Display Properties.
Scroll down the Display Properties until you see Mesh Display
Check off whichever buttons (Edge length, angle, or Face area) applies to what you're interested in.
Select the face/edges that you want to know the dimensions of.