sugarfoot,
You'll learn as time goes by. Was this a kit or ARF? ARF's are typically pretty heavy, but it's also easy for a new builder to build in a LOT of extra weight into a kit. (Too much glue, using heavy wood in the tail...)

I'm willing to bet your Poke would take off fine from a paved runway. GP probably tested this plane on paved runway, and that's where they got the lower engine size.

Virtually ALL small planes have trouble on grass due to small wheels. Unless they're very over powered it ain't gonna' happen. (Try putting those tires on your trainer and see how it does.) Conversely, you could buy large, (light weight) foam tires and try them on your Poke.

You should usually choose from the upper end of recommended engine sizes, especially flying off grass.
Dennis-