I am trying to create a group that has buttons stacked on top of each other like in this picture. This exists in already in the Home Tab
From what I can see from the documentation, these are defined by the Control type element which has three different types: Button, Menu, and MobileButton. Button and Menu create one of the bigger buttons while MobileButton only works for MobileFormFactor.
This feature is not supported with the office web add-in manifests. You could achieve this using COM addins.
You can see office web add-in commands details here, these are the modern javascript add-ins: https://github.com/OfficeDev/office-js-docs-pr/blob/master/docs/design/add-in-commands.md
As it stands in the manifest options today, you can't change the size of the button or stack them. It's a standard button which you can use it as a regular Button or a Menu.
That would be a new feature ask for dynamic ribbon for JS add-in, and I would suggest that you could submit your request in uservoice and upvote for this feature at https://officespdev.uservoice.com/forums/224641-feature-requests-and-feedback?category_id=163563. and upvote for this feature. therefore we could prioritize it based on the vote. thanks for your support
