I wouldn't base you diameter on how much clearance you have.. Size the prop based on hp and displacement and then be sure it fits with room to spare. It's possible to be "too" close to the hull. Seems to me a prop around 12" D would be about max for that boat.
My very similar sized Pearson 28-2 has a 16" prop. That is what it originally came with (2 blade, 16" diameter, 10" pitch). When I switched to a Campbell Sailor they suggested a 14" prop, so I used that for a while. I upgraded to a feathering prop about a month ago, and Featherstream/Darglow recommended going back to a 16" prop.
12" was common for boats that originally came with an A4 (like the Ranger 29), but I think 14" to 16" is appropriate for higher torque/lower RPM diesel motors if one can fit.
I'm not saying that just because my Pearson 28-2 will fit a 16" prop that a Ranger 29 will as well. I'm just saying that it is worth finding out what will fit.
Is there a Ranger-specific mailing list? You might be able to find out exactly what fits over there.