Wanted to know, how big is your under seat bag flying delta? To allow your underseat bag to be securely stored beneath the seat in front of you when traveling with Delta Airlines, it must measure 17" x 13" x 9". This area is perfect for laptop bags, handbags, and compact backpacks. It might be necessary to put your item in the overhead bin or count it as your carry-on if it is larger than these limits. Delta suggests soft-sided bags since they are more adaptable to the space.