of Semtex or C4 will leave a crater and would be difficult to fit into the small cases of the pagers. I'd say a few grams at most, perhaps less.
M-80s had 80 grains (5.2 grams) of mere flash powder in them. The comsumer version only 45 grains (3 grams). Military grade HE is very much stronger than flash powder, though I do not have any numbers handy.