A portion larger than the standard serving; a large helping (commonly used in restaurants to request extra rice/noodles).
ラーメンを大盛りでお願いします。
I'd like the ramen, large please.
この定食はプラス100円でご飯を大盛りにできます。
You can upsize the rice to a large portion for an extra 100 yen.
彼はいつも天丼を大盛りにする。
He always gets his tempura bowl in the large size.