John,

As far as load on the engine goes, going 1" up in diameter is roughly equal to going 2" up in pitch.

Going down in pitch and in diameter have the exact opposite effect, so if you go up 1" in diameter and down 2" in pitch; like when changing from a 12x6 to a 13x4, you are not changing the amount of static prop drag imposed upon the engine.

A given engine will probably spin both sizes at roughly the same RPM.

But (and it is a very big "but"), the static load is not the only one.
A larger prop has a higher inertial load than a smaller prop.
This means increasing the rotational speed of a larger prop take a lot more effort.

Your model is a 3-D capable plane, designed to fly well with a .40-.53 engine and with a 11.5x5 - 12.25x3.75 prop.
Changing the engine to your .60 increased the weight by a significant margin (needing more tail weight does not help either), but this is completely counteracted by increasing the prop disk to 13".

If you increase the prop diameter to 14", the engine will spin at a lower RPM, closer to its peak torque, so thrust is increased.
But in 3-D plane you need an immediate RPM change, which will be harder to get with the larger prop.
Your engine could also overheat, due to a decrease in cooling airflow.

The larger UCD .60 uses a 14x6 with a two-stroke .91. Your smaller UCD doesn't need such a large diameter prop.
I will have to disagree with Joe.

In 3-D more power is great, but not if it comes at such a weight penalty. And it cannot be countered with an even larger prop.

But who knows, maybe the four-stroke will transition the larger prop.