According to these GitHub issue and PR card elements should now have a height property with a "stretch" value, allowing for vertical alignment. I've tried this in Teams (Adaptive Card v1.0) and coudn't get it to work. The Schema Explorer makes no mention of height, although the size property was renamed into width.
Have I missed something? Thanks for your help.
Teams supports the schema here: http://adaptivecards.io/schemas/adaptive-card.json. Or more accurately, since the schema is an evolving document, this specific version https://github.com/Microsoft/AdaptiveCards/blob/998628b3088ad23a0d8fe08934980ffb693c4123/schemas/adaptive-card.json.
That doesn't include the "height" property (indeed I don't think they've updated the schema yet?), so unfortunately, you can't use it for adaptive cards in Teams yet.