Suggestion: Measure your interior (depth, height, width) then take your tape measure to the grocery store with you. Why? Poundage isn't always a perfect indicator of turkey size. The amount of injection added to the bird and its breed, freezing method and sex will affect size, too.
You'll need clearance in any pellet cooker because of the convection effect -- circulation of heated air. I'd say keep the bird sized so that there is at least an inch of clearance at the tightest point and you'll be fine.
For most impact, thaw the bird very thoroughly (slowly in the fridge for several days is best), wash it out and pat it dry -- put it in the cooker cool but with no frozen spots. Set your controller to smoke for about 30 minutes and let it "bask" then kick it on up to 250-275 for an hour or two (slow cook with some smoke) then move on to 350 for the finish (160 degrees in the thigh). Because of the slower cooking, do not stuff the bird. Allow plenty of time so you'll finish early (wrap in foil and place in an empty cooler -- it will stay hot for three or four hours).
I've done a couple of turkeys this way on both of my pellet machines and the results are most enjoyable. You'll get a light smoke flavor and crisp skin.
Hub
