This is the 3902nd most frequent Chinese word.
不大
"Not big" or "not very."
不大 is used to describe the size of the window, meaning 'not big'.
这间房子的窗户不大，阳光进来的量也不多。
The window of this room is not big, so not much sunlight comes in.
不大 is used to describe the quality of the weather, meaning 'not very good'.
今天的天气不大好，总是有点阴沉沉的感觉。
The weather today is not very good, and feels somewhat gloomy.
不大 is used to describe the degree of his happiness, meaning 'not very happy'.