maybe 1cm on the short side of the shim. about 6 inches long, the rest is up to the degree of shim.... that's just a math problem.
in other words, long enough to sit under your ubolts, wide enough to fit the spring leaf, the angle is up to you.
L(6 ") x W(2.5") x RH(1/3") x FH(5/6") @ 3.6 degrees... that's a rise of 1/2" over 6", essentially 1/3" is slightly under 1cm, so your running degree would subtract out the cm over the length of the shim to give roughly 1/2 inch over 6 inches which should be @ 3.6 degrees give or take a degree won't kill you. just make sure they are both the same, and remember it's easier to remove angle than add it back to the shim...
good luck.