Merigold gave good advice!
I made a big oblong thing last year, using a two bulky weight yarns held together.
I used a size 17 needle.
You'll know if your needle is too small if the look of the work is similar to a placemat.
Thick and stiff.
I'd knit a test swatch...tear it down if you don't like the look with that needle...do another...if you like that one...tear it down, too and begin your 'real deal'!