Bot framework composer in web channel gives you file sizes as an array in turn.activity.channelData.attachmentSizes (these don't always line up with the array order of the attachments but it's better than nothing).
However in Teams channel there's no attachment detail other than the url, some download url involving an id, and what the file says its extension is.
In response to this suggestion I tried https://<sharepointsite>/_api/web/getFileByServerRelativeUrl('/path/to/filename.ext')/Properties but it returned 403 Forbidden.
I'm wondering if Graph API is the only way (I'd rather not have to get users to agree to that) or if there's some rest api that can gather it for me?