as above
18" , take it the frame is made for 18" wheels
there are other calipers that look to be shorter than the dc magic , cheapo blank ones seem quite short , a few of the higher end calipers are also shorter , seem to think the dc magics are front specific so perhaps a rear specific one will have long enough slots to reach the 20" rim? rear specific calipers tend to be little more compact.
those post extenders are for v brakes mainly for 1 1/8 to 1.5 rim swaps, or to adjust to suit chain length, don't work with a u brake