I found one on etsy! Just thought I would post on here (even though this is a very VERY old post lol). I've been looking for a printable pdf pattern that was more than one or two sizes. This one has quite a few sizes. https://www.etsy.com/listing/127715071/baby-onesie-pattern-nb-36-months-instant
I found this thread before I found the pattern, so I figured I'd help someone out who might need it.