I taught myself this little trick today. I have already used it a second
time--so now it's a "technique" (which I like):

Put a "dimension" label on an edge of a side before you push or pull a
face, and you can watch the length of the side change "dynamically"
(i.e. as you push/pull).
I guess if you're a super-excellent SU-user (like Swingman or Leon) then
you model your lengths right the first time and won't need to do this
very often (only when you modify an existing model).

Bill