Surprising greatly:
— The dog was capable of astonishing tricks.

synonym: amazing.

Roget 31: great; greater etc. 33; large, considerable, fair, above par; big, huge etc. (large in size) 192; Herculean, cyclopean; ample; abundant; etc. (enough) 639 full, intense, strong, sound, passing, heavy, plenary, deep, high; signal, at its height, in the zenith.    world-wide, widespread, far-famed, extensive; wholesale; many etc. 102.    goodly, noble, precious, mighty; sad, grave, heavy, serious; far gone, arrant, downright; utter, uttermost; crass, gross, arch, profound, intense, consummate; rank, uninitiated, red-hot, desperate; glaring, flagrant, stark staring; thorough-paced, thoroughgoing; roaring, thumping; extraordinary.   ; important etc. 642; unsurpassed etc. (supreme) 33; complete etc. 52.    august, grand, dignified, sublime, majestic etc. (repute) 873.    vast, immense, enormous, extreme; inordinate, excessive, extravagant, exorbitant, outrageous, preposterous, unconscionable, swinging, monstrous, overgrown; towering, stupendous, prodigious, astonishing, incredible; marvelous etc. 870.    unlimited etc. (infinite) 105; unapproachable, unutterable, indescribable, ineffable, unspeakable, inexpressible, beyond expression, fabulous.    undiminished, unabated, unreduced^†, unrestricted.    absolute, positive, stark, decided, unequivocal, essential, perfect, finished.    remarkable, of mark, marked, pointed, veriest; noteworthy; renowned.    ... show more

So surprisingly impressive as to stun or overwhelm:
— Such an enormous response was astonishing.

synonyms: astounding, staggering, stupefying.