If you mean "to stack up" something -- then you should use an
abbreviated mime type motion to dipict what the stacking process
If you mean a two or three feet high stack of something then use
"B" hands (flat hands) or bent-B-hands to literally show the top and
bottom of the stack.
If you mean a stack that extends up from the ground, just use the
bent-hand version (not the index finger version) of
the sign "TALL" to depict how high the
stack is (or would be).
If you mean "stack" as in a stack of 100 dollar bills you can use a version
of the sign for "thick."
THICK (Version: The thickness of something horizontal) (Facial expression
here indicates "very" thick)
Now, if you are talking about a stack of ordinary paper (not a stack of
something impressive like $100 bills) then you don't need to exaggerate your
facial expression so much eh? (Unless you get excited about stacks of
"Stack" as in a "relatively thick" stack of something.
The facial expression for the above sign as based on showing a "relatively
thick stack of papers." That mouth morpheme you are seeing means "fairly,"
"relatively," "substantial but not overwhelming." You might also see
it on signs like 'so-so."