My dog got the wrong size Nexgard. Can I cut the chew in half?

With medications in tablet form, you can certainly cut them in half and still be sure that you are giving half of the medication. Unfortunately with chewable medications (like Nexgard, Heartgard, etc.), splitting a dose in half does not ensure that half of the medication will be given. The active ingredients are mixed throughout the chews, but not necessarily in a uniform way. To answer your question, no, you shouldn't give Frosty half of a Nexgard. Instead, see if you can exchange the product for the correct size. It's likely that your vet will be able to prescribe a single dose while you sort out the exchange process. I hope that everything goes well!