I'm the distributor for Iowa, so obviously with a biosness, I'll let those on here that have purchased one comment on their performance.
As for the sizes, here are the recommendations:
PM300, 3ft ATV 300-500cc
PM400, 4ft ATV 500cc or bigger
PM400, 4ft Tractor 25-30hp
PM600, 6ft Tractor 35hp or bigger
Hope this helps, let me know if you have any other questions.