Unsmall

One can cure oneself of the "not un-" formation by memorizing this sentence: "A not unblack dog was chasing a not unsmall rabbit across a not ungreen field."

Source: tatoeba (3469584)

Bill parked at a drive-in beside a big Buick. Its lone occupant, an unsmall woman, looked over and down at the sardined Simea, and grinned broadly.

Source: wiktionary

Jeremy Kagan, twenty-six, is proud owner of a Harvard degree (Phi Beta Kappa), a beard (brown and bushy), and an unsmall ego (amply justified).

Source: wiktionary

Another option is to save receipts and obtain a refund at the airport, although the commission charged is unsmall.

Source: wiktionary