Spirit cabin baggage (cross-checked from aggregator summaries)

What the official page says

Personal item (free on every fare): up to 18 x 14 x 8 in (45 x 35 x 20 cm), including handles and wheels, must fit under the seat. Carry-on bag (paid add-on on most fares; included on top bundle): up to 22 x 18 x 10 in (56 x 46 x 25 cm), including handles and wheels. No published weight limit for either.