试衣服该怎么说英语

To try on clothes in English, you can say "I want to try this on" or "Can I try this on?" If you are in a store, you can also ask the salesperson for a fitting room. You can say "Can I try this on in the fitting room?" or "Where is the fitting room?"
Once you are in the fitting room, you can say "This doesn't fit" or "This is too big/small" if you need a different size. If you are trying on a top, you can say "I need a bigger/smaller size in the bust/waist/arms" to specify which part of the garment doesn't fit well.
If you are trying on a bottom, you can say "I need a bigger/smaller size in the waist/hips/legs" to specify which part of the garment doesn't fit well.
I hope this helps! Let me know if you have any other questions.