To upsize is to make something bigger, or to become bigger. You can upsize a drink at a fast-food counter, upsize a car, upsize a house when the family grows. A company can upsize when it hires more people. It works both ways: something can upsize on its own ('the business upsized last year'), or you can upsize it ('we upsized the order'). This is a fairly new word and it wears its origins on its sleeve. It was built as the opposite of 'downsize' — and 'downsize' came first. That matters, because 'downsize' is the famous one. In business English, downsizing means cutting jobs, and everybody knows it. Upsize was then made to fill the other side. So the word still sounds a little bit like business or advertising language, not like something you would find in an old novel. The fast-food use is the one your students will meet most: 'Would you like to upsize that?' means 'do you want the bigger portion, for a bit more money?' In Taiwan the same idea is usually offered as 加大. Note that it is not really used for abstract things. You upsize objects, portions, vehicles, homes, workforces — physical, countable things — not feelings or plans.

加大、擴大（飲料、餐點加大；房子、公司規模變大）。速食店的「加大」就是這個字