I. su ‧ per ‧ size 1 /ˈsuːpəsaɪz $ -pər-/ BrE AmE adjective [only before noun] American English
a supersize drink or meal in a ↑ fast food restaurant is the largest size that the restaurant serves:
Could I get a supersize fries with that?
II. supersize 2 BrE AmE verb [transitive] American English
to give someone a larger meal or drink in a ↑ fast food restaurant