Perfectly round; completely circular or spherical—used to describe something that is neatly round.
夜空に真ん丸い月が浮かんでいる。
A perfectly round moon is hanging in the night sky.
子どもの頬が真ん丸いので、つい頬をつねりたくなる。
The child's cheeks are so round that you just want to pinch them.
ガラス玉は真ん丸くてとてもきれいだ。
The glass marble is perfectly round and very pretty.