Actually the KPI object only provide 3 Font size options : Small, Medium, Large. They works when using the "Responsive" and "Fluid" layout behavior even with small sized objects of using a grid of 3x2, but because of the responsive behavior, when we have several KPIs with numbers of several lengths, the size of the font is not consistent between them.
So in this situation, we can use the fixed layout and keep the font with same size on all KPI objects, but when creating objects with small size, like 3 x 2 grid blocks, the small size is not small enough. See example using Responsive and Large Font on the left and Fixed and Small Font on the right.

The value proposition is to offer more options to developer create a dashboard with consistent design.
Add more options for small font sizes (Tiny, Narrow, Extra-Small) or allow user to set a font size number if Fixed Layout is chosen.