Last tested: May 2021
Single value visualisations do auto-size, however all single value visualisations on a dashboard will have the same font size. So if you have some tiny tiles next to some large tiles, the font size will scale to match the tiny tiles.
Also, if there is more than just the value in the visualisation (e.g. the "Comparison" label in the visualisation settings) then that can make the font size smaller.
Make sure that the "Comparison label" in the visualisation settings is off.
Enlarge the tile size of the smallest tile on the dashboard, which should increase the font size of the other single value visualisations
Use HTML on the LookML field as in this community link.
Vote for this request in our Feedback system
Example of Expected Behaviour