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.
Further thought: It would be easy to disguise the explosive as an electronic component such as a resistor or capacitor. This would pass a cursory inspection.
The explosive in the pagers was PETN, pedestrian and in use for more than a hundred years.