Hello is Frontier airlines strict with personal item sizing? Thank you!

strict. that is how they make their money. bags and carry on normally cost more than base airfare. you won't get a rollerbag onboard but may get a slightly non conforming backpack since when they are busy they don't make every passenger use the sizer at boarding.

they post huge signs at gate and now make anyone who paid for carry on get zone1, that way when it is higher zone #s turn to board they can be on the lookout.

in flying them weekly they shame passengers with any kind of a rollerbag into paying at airport or on app. to the point I have seem passengers passengers turn on the tears at the counter or gate (pretending to not know the policy despite many prompts and warnings on app, website, at checkin, even OTAs).

Will they still charge me if my backpack is a bit larger than dimensions given?Thanks!

Probably not, if it's not obvious. They seem to focus on roller bags and other more traditional luggage. But they can always make you stuff it in the sizer.

I don't think I have ever seen them check a normal size backpack (I wouldn't try the huge ones backpackers use), its always roller bags. Same on Spirit. Normal backpacks are harder to "catch" people with anyways, you can just shove it in the sizer and chances are good any of the dimensions you are off on can just be squeezed in. Any bag with a frame obviously you can't do that.

My recent experience in boarding in PHX and DEN is similar to what has been stated. I bought a carry on, so I was in zone 1. I was concerned about my normal personal item being too large, so I downsized my normal travel backpack into a small bag which fit the personal item dimensions. In this case, it was not necessary. I think as long as I purchased a carry on, no one was too concerned. But, you never know.