no one needs a flame suit, but the OP needs to be more accurate on what really determines the proper plug in an engine of any disc. saying that short plugs are for disc. from 0-.15 and long plugs are for anything over .15 is in error.
some racing engines of .40 disc. have required short plugs. normally as a start, the end of the plug should not protrude past the end of the threads in the head..........adjustment to cold,medium, and hot plugs follows.

as has been posted though, variables in fuel, altitude, temp and humidity conditions will rule the requirment of what plug is actually best............very tedious testing will find the end result, but since not many modelers have the equipment or knowledge to measure rpm in an accurate manner, and even more so know what detonation looks like, it's better to just go with what a manufacturer recommends for fuel,prop, and glow plug.
i have a hard time running OS plugs in some larger engines as they really aren't short or long which leaves them short of the threads in heads that are really in need of long plugs. they may do an OK job, but they aren't the ultimate even for their own engines.

Rossi plugs are much better plugs and they do make a difference.