There is a difference between fit and suit. Fit refers to size and shape. When your clothes fit you, they are neither too tight nor too loose for you.
- This shirt doesn’t fit me. Have you got a smaller size?
Fit can also mean suitable or good enough.
- Is this dress fit to wear?
If a person is fit, he / she is in good health.
- A sportsman must be fit.
Suit refers to style, colour etc. If something suits you, it is convenient or appropriate for you.
- This hat doesn’t fit me. (= It is either big or small for me.)
- This hat doesn’t suit me. (= This hat isn’t appropriate for my sense of style.)
- Bright colours don’t suit me well. (= NOT Bright colours don’t fit me well.)
- The cold climate in Kashmir doesn’t suit me. (= The cold climate in Kashmir isn’t good for me.)
Suit can also be used to say whether arrangements are convenient.
- Will Tuesday suit you? (= Will Tuesday be convenient for you?)