Issue
Adaptive card is not using the whole width of the mail window and is only showing as a narrow card, using maybe 40% of the window width.
Resolution
There is no fix for this. All visual aspects, width, height, font-styles, colors etc are controlled by the "host app" the host app is the channel eg MS Teams you are posting your card to.
See here for details: https://docs.microsoft.com/en-us/adaptive-cards/rendering-cards/host-config
Only the hosting app can define things like width.
