You can change the size of the dominant hand circle by changing the value in row 1 and column 3 of the target table. The Show target block is using the columns [4 4 3 3]. This is a simple task which shows how to change the hand feedback from the default white circles.
Technically this could be switched to [4 0 0 3] and it would look the same. If you look at the help for the show target block you can see that for a circle the values here mean: fill color column, stroke color column, stroke width column, radius column.
Task last tested in Dexterit-E 3.10 and MATLAB 2015a.