http://www.dalekeller.net/atm/newton...t/newtsoft.htm
Check out the program on this page - It will help you get your design right.
Field of view is limited by "clipping" the light available to the eyepiece. This can be done by:
1 Having too narrow a tube,
2 Having too small a diagonal or putting it in the wrong place,
3 Using too small a focusser (avoid old 1.25" units).
It's a ballancing act, but easy enough when you follow the rules.