I wondered if the bag sizer boxes at the airport (or whatever one calls them) are spot on or if they provide any wiggle room at all?

Basically I have been looking at the Travelpro Maxlite Tote that is supposedly 16x13x8 but I've seen on Amazon that some people reported it measures 8.5" instead of 8" after the wheels are factored - so I wondered if anyone has actually ever measured the sizer box to see if the dimensions are exact.

This will be my first time flying Frontier. I think I'm also going to pay $20 and invest in an REI Sit Pad for the hard seats I keep hearing about! It's a 5 hour flight.

Thank you for any help or advice!