A standard or normal-sized serving at a restaurant (not large or small).
ラーメンは普通盛りでお願いします。
I'll have the regular (normal-sized) ramen, please.
彼は大食いだけど、今日は普通盛りにしておいた。
He's a big eater, but today he went with a regular portion.
定食は普通盛りが一番コスパがいいと思う。
I think the regular set meal gives the best value for money.