This seems like it's already posted somewhere, but I'll just throw it out here anyway. This code allows you to calculate the total lengths of multilines segregated by their multiline size/scale.
In order to use the attached LISP code, initiate the APPLOAD command from AutoCAD, and load this file. Once you've done so, you'll notice that you'll have a new command TML.
To use the TML command:
Select all the multilines that you'd like to calculate for its lengths.
Click a point somewhere and an MTEXT will be created with the details for the lengths of the multilines for each size.
Feel free to add any ideas, thoughts, criticisms, etc... and I'll try to modify them.