Scale size on the screen depends on the size of the game window.
Usually I think in %, depending on the scale chosen and on the green speed. On a 30 ft scale and standard greens, 20'3" would be about 68%. There are printable putter scales to support this calculation and the putter pal from the pro shop helping to see the %%. You might fix a paper ruler at your screen.
Don't forget to look into the Putting Tips over there ==>