Global sizes tab is used to set all ad slot which will be available for all publishers.
Internal ATF Ad Name - is the name of the size, used in Overviews screens and to group similar sizes. Global size has to have Ad Name
Width & height - dimensions of the ad
Fluid size toggle - to turn on or off if it is a fluid size: https://support.google.com/admanager/answer/9178980?hl=en In case it is a Fluid, no dimensions need to be specified
Format - select 1 from the list
Note - select 1 from the list
Add child size - new size under same Internal ATF Ad Name. Child sizes can have different parameters than parent size.
In use - Label shows which sides uses this size