Spent countless hours trying to find workarounds with no luck. Would love to have this quality of life option.

It would save so much time to be able to add to/edit the default artboard dimension options. I have never and probably will never need both Apple Watch artboard dimension options, but basic common print sizes A0-A6 (if not all the way to A10) should be easy default artboard options as they are commonly used by many designers, both when opening a file and when creating new artboards in already open files. This should be really basic & easy and would make me very happy, thank you!

Illustrator needs this urgently, photoshop has had a similar feature for a while, it's a shame illustrator doesn't have the same!

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!

Hello again, Hans.

'There is no clear way to do that’ means that Illustrator does not provide a clear way to include our own presets to this list.
To have something appear in Templates, you have to put in in the designated folder, something like %AppData%\Adobe\Adobe Illustrator 26 Settings\en_US\x64\Templates

'Why can't you just add common social media templates as well' — I can’t add anything, I am not an engineer, I am a mere user with the badge and some knowledge :)

The Saved tab is not meant for user-saved presets... yeah, it’s stupid, I know. It supposed to have downloaded and imported templates.
The help says: 'Quickly access templates that you've downloaded from Adobe Stock (Saved tab)’ — https://helpx.adobe.com/illustrator/using/create-documents.html

So it’s not the functionality that is missing, but the name is wrong and misleading.

Hi Egor. Not sure what you mean with "there is no clear way to do that"? You already have blank document presents like A4, Poster etc. Why can't you just add common social media templates as well?

Another thing, the saved tab makes no sense because when I create a custom template and save it, it does not appear in the saved tab.

We need our custom templates to appear when we want to create a new document.

There is no clear way to do that, but you can save a prepared document as template and later quickly create a new one using File > New from Template.
Not the perfect solution, but does it solve the problem for you?

Adding size presets for specific platforms like Instagram, Facebook, Twitter, etc. would be helpful to easily create posts, profile banners, and story graphics

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.

update:

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)

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.

Voted for A5 in preset size.

same here. another reason to get annoyed with Ai for years, can u guys fix up all these little annoying things?

Yes, thoroughly agree but also please make it easier to change the presets units to eg millimetres too! Who honestly uses points as a page dimension unit in most of the world? Is that an American thing Adobe?!

And allow us to change the default measurements on them into other units eg millimetres!

2021 and still annoyingly lacking A family preset sizes

Yes please, A1-A7, thanks! @adobe

Can we please have the ability to add our own presets in categories we select? Thanks...

It would be so nice to be able to create blanc templates (not only A5 but every custom format) an put it in the wanted section like «print» | «web» | «mobile» etc…
Like this.

While Adobe does not add new templates I use the following workaround:
Create a new document with desired dimensions (once) -> Save as template -> somwhere_you_like/(MY_TEMPLATE.ait) -> move this file to /Applications/Adobe Illustrator XX/Cool Extras/en_GB <or your_language>/Templates/Blank Templates/ (requires password).
Then Illustrator -> New File From Template (shift-command-N) and voilà.

sigghhhhhh... looks like Adobe still hasn't bothered fixing this issue then, even with the latest update I've just done. Its such a basic essential option too!