Most commonly a 13/16", but a 3/4" is possible.
The spark will depend upon the type of engine being used, either 2 cycle or 4 cycle. If it is a 4 cycle, chances are the engine was produced by either Tecumseh or Briggs.
.030
With a wrench
An Autolite 258 is the recommended spark plug for a 6. 5 horsepower Tecumseh engine. A Champion RJ19LM also is recommended by the manufacturer.
The correct spark plug for a 5 hp. Tecumsen 4-cycle motor is Champion J-8 (RJ17LM). The plug gap is 0.030 in. (0.76 mm)
My book for my John Deere snowblower , which has a 5 horsepower Tecumseh engine , shows to use the Champion RJ17LM spark plug
Champion J8C
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Set the spark plug gap to .030"(.762mm).
Autolite 258
.030 inch ( on my 24 year old 5.0 horsepower Tecumseh engine )
Two cycle (two Stroke) engines generally have one spark plug per cylinder. So a four cylinder, 2 cycle engine will have 4 spark plugs.