I have a 16.9 in laptop and I have a Targus brand that shoulder bag. It always fits. The rolling variety is usually too thick to fit under the seat, especially if you are flying on a plane with 50 or fewer seats. I suggest putting your clothes etc in a rolling suitcase, and hanging the shoulder laptop case over the suitcase handle as you are traveling through the airport. You will still be within your carry on baggage limit and if they have to check your suitcase your laptop will easily fit under the seat. Its just an easier way to travel for me and I never have to worry about my bag not fitting.

I happen to agree with [Flight Attendant] as a traveler both Domestic and International it is always more convenient to separate the computer from [a] roller type bag. You are rarely in control of luggage, hand bags go where you go! In addition regarding security screening, again more convenient to already have it available. Happy Travels!