To do this properly, you should probably run through a full set of calcs.  Dave Gerr's Propeller Handbook is very approachable and with few evenings work you could work through all the compromises of propeller choice.  It would be interesting to know how many blades and what is the pitch of your current propeller.  Too little blade area can lead to the blade loading getting too high with subsequent cavitation and loss of lift (thrust).  So, generally speaking a larger diameter prop with more blades will give you higher thrust and should keep the blade loading low enough to avoid cavitation and the concomitant loss of thrust.  However, there are lots of other factors to consider such as maintaining adequate clearance from the hull, enough pitch to reach cruising speed, and how efficient the prop will be at cruising speeds.  Gerr clearly explains how to figure out all these bits.

I have no affiliation with Gerr or his publisher, I just like erudite authors and informative books.

My boat is 30' LOA, 23' LWL, 10'7"" beam, displacement 10, 300 pounds.  The engine is a Mitsubishi (Vetus), 25HP at 3, 600 rpm. Gearbox name plate gives gearing as; iA=2.05 [opposite rotation to engine]; iB=1.86 [same rotation as engine].  I want to fit a feathering or folding three- (or four-?) blade propeller which will best possible upwind and upsea punching power, within the parameters of my power, reduction gear, and displacement. I would have thought that 25HP should give a vessel of my size a reasonable punching-power in a reasonable seaway and/or breeze.

the present feathering propeller is 13.5 inches diameter, which is okay in smooth wsater but has no punching power at all.

What diameter of propeller should I be looking at? (Or should I be looking at changing the gear box to another reduction gear ratio?)

Or what?