Not big, not small — somewhere in between. It is a plain, everyday describing word, and you put it in front of a noun: a middle-sized dog, a middle-sized town, a middle-sized box. People use it when the exact size does not matter and they just want to place the thing in the middle of a range they and the listener both have in mind. That range is never stated, so 'middle-sized' means different things in different sentences: a middle-sized dog is maybe knee-high, a middle-sized country has maybe twenty or thirty million people. The listener fills in the scale. It sounds a little old-fashioned and a little homey now — you meet it more in storybooks and in older people's speech than in the news. In business or writing about companies, people say 'medium-sized' instead ('medium-sized businesses'), and that is much more common today. You will also see it written 'middle sized' without the line, but with the line is safer. It is used only before a noun; you would not normally say 'the dog is middle-sized' — you would say 'the dog is medium-sized' or just 'it's not very big'.