I've never used size 6 diapers. Even my oldest son (who was big for his age and finally got the potty down by age 3) didn't need beyond a size 5. We then used a nighttime pull-up for a bit. Middle son was fully trained while still in size 4 diapers.
same here.. full trained by size 4, but still use nighttime size 5s .. i wouldn't get size 6 unless you're OK with donating them