I wouldn't think the precise size is hugely critical, but ISTR from my aero-modelling days that a 1.5cc engine would take a prop of 7" to 8" dia and around 5" pitch. The larger diameter might give better starting dur to flywheel effect and also keep the revs down on a new engine. I don't think that you'll do any harm at around those sizes.

Hi Bernard – still here in Beccles

Clive's advice is sound – early runs on say a 8 x 4 and a 7 x 4 for general running will be fine. Keeping the pitch at 4" or below allows the engine to rev without being loaded. You could use a 6" pitch on bench running but that would usually be much better suited to flying situations. I very rarely (bench) run my engines on 6" pitch props for that reason

I have a FROG Talisman control liner with a Frog 150 in it. So nippy it's getting difficult to handle these days so it flies on a 7 x 4 prop to slow it down a bit

Getting small props may prove difficult – so much these days are aimed at electric motors but Leeds Model Shop might be a good start.

Just to get you in the mood here's a new model – it's an enlarged 30" span KK Ranger with an OS 20FP glo motor converted to diesel.

See you at Lowmex?

Best – Tug