General Negation:
* Mediocre
* Ordinary
* Average
* Unremarkable
* Dull
Specific to Size/Scale:
* Tiny
* Minuscule
* Insignificant
* Small
Specific to Quality/Impact:
* Terrible
* Awful
* Poor
* Unimpressive
* Disappointing
The best antonym will depend on the context in which "stupendous" is being used.