Help me decide which size pillow to get for my toddler?

I am getting my 15 months son his first pillow. I was thinking about getting him a travel size one, which is 12.5" X 16". Would that be good untill he’s 5 or older, or should I just get him a regular size pillow with a lite filling, which is 20" X 25"?
I am getting him his own pillow because he keeps trying to steal mine (sleep with his head on mine, we co-sleep).
I cant let him decide, because I am ordering his pillow online. For a pillowcase, I can just sew him one.


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