An ephemeris of the sun will give you the current diameter
of the sun in arcsec from the following site:
If you have an observation of the partial limb of the sun as
shown below, one can determine the diameter of the sun in
pixels by using three spaced points (A, B, & C) on the limb. Knowing
both the diameter of the sun in arcsec and pixels allows the
plate scale to be found. The positions of A, B, and C can be
determined in rectangular coordinates from a number of programs.
The linear algebra scilab is free and can be used to make the
computation whose output is seen below. I've printed out a few
other things such as the nominal 0.5x focal reducer which is
really operating at 0.42x on these observations as well as
the f number of the system.