What is the idiomatic way to say "clothing fits"? There are various ways to express the verb "fit" in Mandarin, such as 適合，配合，對 and so on. I'm not sure which one to use when expressing sentences like:


  The clothes no longer fit me.
  I need clothes that fit me.


The idea is that the clothes are the wrong size now, there is nothing else wrong with them. (不)適合我 seems like it could also mean "I (don't) like the style".  

Would I even use a word like "適合" here, or an expression like 穿不上? Is it different if the clothes are too big instead of too small?
I am a native Chinese speaker and glad to help you.
In Chinese, there is a more idiomatic way to express that the clothes do not fit.
You can simply say "合身".
As an example: 这件衣服不合身, 它太大(或小)了.
(Translation: These clothes do not fit me because they are too large (or small).

When you use "适合", a misunderstanding may occur. This word can also be used to describe how the clothes look on you. (Just as your example: you use it to say you don't like the style.
As an example: 这件衣服不适合我, 它显得太年轻了.
This means these clothes are often preferred by younger people.

When you say: 这件衣服穿不上
You only emphasize that these clothes are too small for you.