Monstrous, prodigious, tremendous, stupendous, monumental are comparable especially in their extended more or less hyperbolical senses in which they mean astonishingly impressive.
Monstrous commonly applies to something abnormal, usually in actual or relative size, but often also in shape or character; the term frequently carries suggestions of deformity, extreme ugliness, or fabulousness.
Prodigious usually implies a marvelousness that exceeds belief; it sometimes applies to something entirely out of proportion to what is the previous or usual best, greatest, or largest.
Tremendous may come closer to awe-inspiring or terrifying in its immensity than to gigantic or enormous, its common denotations in more literal use.
Stupendous implies the power to stun or astound; it describes something that because of its size, its numbers, its complexity, or its greatness exceeds one's power to describe or explain.
Monumental in its extended sense (see also MASSIVE ) applies to something as conspicuously impressive or as massively framed or constructed as such a monument as a great cathedral or an impressive memorial.