[大盛り] / [中盛り] / [小盛り]

oomori / chuumori / komori

large / middle / small

"大（oo/ dai）", "中（chuu）" and "小（shou）" mean

"large", "middle", and "small" respectively.

And "盛り(mori)", which is derived from verb "盛る(moru)",

means "to fill a bowl with rice".