I take it this is a DIY Newtonian?
If you think your secondary is to small the only way around this with the same secondary will be to lower the profile of your focuser so the image plane is closer to the optical center line.
Then you will need to move the mirror back to attain focus.
Most probably cheaper and simpler to get a larger secondary than buy a low profile focuser and moving the mirror.
Do you have a copy of newt2.5 , if not down load it now and feed the figures in.
Read the help files on scope design first to understand the paremeters and terminology.
It's not a difficult program but will give you all the figures you need to see if your in the ball park.
You want to obtain a fully illuminated field of a bout half the diameter of the largest eyepiece field lens (bottom lens) you have or a bit bigger.
The 75% illumination should be the same size as or larger than that field lens.
If the scope is soley for CCD imaging than you need to fully illuminate the diagonal of that CCD chip.
If you want as small as diagonal as possible for a planetary Newtonian carefull designing and using a truss tube and 25mm low profile focuser will get quite a small secondary of under 20% and maintain a fully illuminated field big enough for even a 31mm Nagler.