Folks,
We need to take the advice on prop lengths with a grain of salt. There are a number of details that should be noted beyond just their prop size.

It really appears that the ARF comes with shorter gear than the BIY kit aircraft comes with. And when someone suggests they're using a longer prop, it'd be worth asking if they're using the stock gear with the stock wheels.

Engine size dictates prop size. Prop size affects prop clearance. Prop clearance can be increased by bending your stock gear taller or retrofitting longer gear or using something that will pull the gear legs in or using larger wheels.

There is a BIY Skybolt. It appears to come with taller gear than the ARF. The ARF appears to come with gear that when unmodified will only give a 13" prop about 2 1/2" clearance to pavement.

When considering the engine size for your Skybolt, it'd be sensible to ask yourself, "How deep is the grass at the field?" and "do I want to rebuild the gear?"