The old manuals say 0.035", but the Moyer Marine archives have some postings talking about 0.030" for some plugs.
Atomic 4 Parts Catalogue and Operator Manual
westerbeke, marine generators, engines, Official Site
or to to the Westerbeke main page and find Atomic 4 in the drop down list of products.
The Moyer Marine forum archives are full of useful info about plugs and everything else A4.