I work on a windows laptop (X1 carbon, windows 10) and I've been struggling with a tiny dashboard canvas to work with (see picture below). It looks perfectly normal (fills most of the gray space) on a colleague's mac. Once I do manage to squish the components in, it looks normal on Tableau online.
If it's a 4K screen then your only choice is the reduce the screen resolution. I don't think there's any other solution yet.