Carry-on bags shouldn't be larger than 22 inches long, 14 inches wide and 9 inches high including handles and wheels. Dimensions of personal items should not exceed 18 inches long, 14 inches wide and 8 inches high.
Can a full backpack be a personal item?
In most cases, backpacks are considered personal items as long as they fit under the seat in front of you. For example, if you use a backpack as a personal item and put it under the seat in front of you, you can then use a duffel as a carry-on bag as long as it's under the airline's size requirements.