The full sentence is below and the context is a guy, describing someone he just met.
I don't know what the 「な」 is doing/means.
As already answered in the comment, "出過ぎず、足りなすぎず" as a whole is used as a na-adjective.
出過ぎず足りなすぎない完璧な躰 would be also fine and more "standard", but in this case 出過ぎず足りなすぎず is a kind of set phrase, so probably the author did not want to conjugate it.
- 早い朝食 early breakfast
- 早くな朝食 (incorrect)
- 早くもなく遅くもない朝食 (standard)
- 早くもなく遅くもなくな朝食 (perhaps nonstandard, but OK) not-so-early-but-not-so-late breakfast