Have an option to add your OWN sizes to the New Document Window
Add an option to the "Saved" tab on the "New Document Window" to add your own Template/Documents. i.e I want to add a blank template for A5 size and always have it handy when I create a New Document.
Steven Scott commented
The whole point in presetting templates is to aid productivity, if you can't add your own presetting what is the point in even calling it such. Another reason for me to question why the **** am I paying for your software!
This is pretty easily do-able in the legacy new document dialog.
Add documents to this folder to make them available in the drop-down list:
C:\Users\YourUserName\AppData\Roaming\Adobe\Adobe Illustrator 26 Settings\en_US\x64\New Document Profiles
If you want to make your own templates (*.ait files) and have them available via the "Templates...", save them here:
C:\Program Files\Adobe\Adobe Illustrator 2022\Cool Extras\en_US\Templates
locations will be different on OSX, they may be in a hidden library folder somewhere. The directory structure should be very similar though.
The new document profiles location on OSX *might* be here:
Hard Drive\Users:[user name]\Library:Application Support\Adobe:Adobe Illustrator 26\[language]\New Document Profiles
this is from a site posting instructions on how-to for CS5, with notes about updates for CC versions up to illustrator 2015 (version 20)