I'm an amateur looking for a laptop bag Also looking to take advantage of cyber monday deals.

My main question is, how big can it be to still fit under the seat in front of me? It's important to me that it counts as a personal item because I hate checking bags (so I want to use my spinner carry-on, plus a laptop bag)

Is 5" thick too much? The one I'm considering getting is 16.5" x 12" x 5" (this one).

I usually fly VX if that's important (but I do want to be able to fly other airlines!)