@peterose and @emilystewart30 our small and large intestines are not rigid - I would imagine they expand with any contents that pass through them.

A tablespoonful (equal to three teaspoonsful) is not likely to cause blockage as long as you drink another glass of water after the first one with the Metamucil/psyllium fibre. Drinking that second glass of water is more important, because not doing so could cause a blockage.