Atlassian has not implemented the ability to resize macros that display embedded content from attachments in Cloud.
You can force the draw.io diagram macro to display your diagram at a different size by changing the Zoom value in the format panel when editing your diagram in draw.io.
Fore more general info on draw.io for Confluence or Jira, please refer to our draw.io webpage.
And some great blogposts! Blog - draw.io