I would make it the size it comes out as .. I personally prefer sq quilts but thats my choice.
It will come together very quickly once youve cut the pieces ... the hardest part will be deciding on the 'pattern'
Couple of
Linkys if youre interested
It doesnt need to be 1/4" seam if you're worried about it, your normal setting + small stitch length (2 ?) will do as long as you factor that into your cutting measurements and its consistent throughout the whole project.
And Im always on the end of the phone if you have a query .. tho you're not exactly a beginner!
ETA ... 100% cotton or bamboo if you can afford it and want it. Please stay away from the poly ... my opinion only!