Sizing Secondary Mirror

I am using the Newtonian Telescope Design Planner available on the web to design my 12.5" f/5 lightweight dob. For sizing the secondary mirror, you have to select the desired minimum 100% field illumination, either measured as a diameter in millimeters or as an angle in arcminutes. They suggest a minimum 6 millimeter or 13 arcmin 100% field illumination. My question is: what value should I use? Of course, I will have a 2" focuser and low power widefield eyepieces. Any help will be appreciated.

13 arcmin of full illumination is a little small if you're planning on using 2-incher wide field eyepieces. You could get away with less with smaller eyepieces or higher powers. With a 35MM wide-field type eyepiece you're going to have a field over a degree across, somewhere in the 60-100 arcmin range. You don't have to go fully illuminated all the way to the edge -- how big you choose to make the fully-illuminated field can depend on what percentage of illumination you're willing to tolerate at the periphery of the field.