Print a 25mm cube.
Note which orientation (arrow for front or similar, otherwise it is hard to figure out which is X and which is Y direction).
Measure with calipers to get exact dimension.
In the EEPROM menu is a number of steps per mm for X, Y, Z and Extruder. (If you have done the extruder calibration you have used this menu).
Alter the X and Y steps based on the cube dimension. e.g. if the cube should be 25mm and is actually 20, then your steps needs to go up about 25%. Existing steps / (Measured dimension / Required dimension).
Give it another slice and print, should be closer.
The multiplier impacts the calculation of filament diameter, nozzle size and required wall thickness (and may vary from filament batch to batch, based on other forum entries).
If you print the cube single wall then measure the wall thickness, it should be the 0.42 in the slicer settings. If the thickness is out, the multiplier is a "fine tune" to get the calculation a bit closer. If you measure the thread, you may find it is actually closer to 1.65 (or even 1.6). Between that setting, the extruder steps and the multiplier, you should be good for fairly accurate plastic extrusion. Then the X and Y gets you accurate sizes on the model.