Posted By Jeff Blazey
As diagonals and holders are relatively cheap, get two or three very high quality ones. I suggest a 1.25" or 1.33" minor axis and a 1.00" and just try them out. That's roughly 21-22% and 16% respectively central obstruction by diameter. The 1.00" diameter diagonal can give great, high contrast high power results and I've found that light drop off at the edge is gradual and the eye is not that sensitive to it. But if you truly insist on a big fully illuminated field, also try a 1.5".
"We learn by doing and learn best by screwing up in the process. Well, I've learned a lot. "