We have used the fuzzibunz MCNs and DD rolled at 3 weeks (only tummy to back though, back to tummy took way longer). The ones we used weren't one size fits all and we have used 2 different sizes - Small when DD was a newborn - 14 months and since then Petite Toddler as she is quite slim in build.

I can't say that we saw any movement restriction or any delay in achieveing physcial milestones (compared to her sposie counterparts) but we did make sure that we gave her at least 30min nappy free time every day so that may have helped.

I agree to have a lok around at some slimmer makes as this may suit bubs build better. There seems to be a wealth of experience on this site so I'm sure you will get heaps of recommendations. As for fuzzibunz, I'm not sure if you can get them in Australia anymore but I found them to be brilliant and as they are pockets you can keep them slim for day and bulk them up for night.