if this bike is just a cruiser, gear the hell out of it because there's nothing wrong with standing up and hammering if there are any climbs. it's a lot more fun than spinning out on all the straightaways. on my trail bike (hardtail) i run 32x15. ideally, i would run it a bit shorter but i don't want to switch ring gears, and a 15t cog is as small as i can go with my setup.
i don't know if i'd want to ride a 40x16 on the trails, but i also don't know what kind of terrain you plan on riding it. if it really is going to be on the "easy" stuff, i'd recommend trying the 40x16 first, then switch to 38x16 if it's too difficult, but that's not the mentality behind why we ride SS in the first place.
we don't ride SS to be easy! (though admittedly, me on a SS seems a bit easier than many of my friends on a geared full-suspension MTB!)