Pre-heat help please

I own an RV-7 with a carbed O-360 and standard snorkeled cowl. I am trying to buy some supplies at Home Depot to route my heater into my cowl. I was wondering if anyone knew how big of a duct tube would be appropriate for either the exhaust exit on the cowl, or either of the inlets next to the prop. Also, is there any reason I would choose one location over the other? It seems like the exhaust exit would be easier and I wouldn't have to worry about scratching my cowl paint. It is a forced air propane heater I am using.

Thanks. Andy

Don't know exactly but...

Hot air rises so exhaust portal is what I use. Also mine is a 1100W (or maybe 1500W) electric space heater to which I've attached a piece of 4-inch flex metal dryer ducting. Don't know how easily that can be done with your propane unit but probably not too difficult.
Patrick

Thanks. The heater is round so a step-down ring should do the trick. I just don't know what size hose I need yet. Sounds like 4" works for you, so maybe that is the ticket. Thanks.