When you are shopping for clothes you need to see if they fit properly. To do this say, “Can I try this/these on?” to the shop assistant.
Whether you use ‘this’ or ‘these’ depends on if there is more than one. If there is more than one, say ‘these’. Remember that trousers/pants/jeans/etc. are talked about as if they are plural. If you want to talk about them in the singular form, say ‘this pair of trousers/pants/jeans/etc.’
After you have tried your clothes on, they will ask, “Is it OK?” or “Is everything all right?”
If you like the clothes, say, “Yes, I’ll take it/them.”
If the clothes don’t fit, say, “It’s/they’re too tight/baggy.” or “It’s/they’re not big/small enough.”
If the clothes fit but you don’t like them, say, “I don’t think the colour suits me.”