、。
Don't worry — it'll be a perfect fit on you, no doubt about it.
Literal
It's-fine, you [if/as-for-なら] just-fit no-mistake [assertive-よ].
A casual reassurance. ~なら is the contextual conditional used as 'as for you / if it's you,' framing the rest of the sentence as specifically about the listener. ジャストフィット is wasei-eigo ('Japanese-made English') for 'perfect fit' — a compound coined inside Japanese from English parts, not borrowed as-is from English. 間違いない ('there's no mistake') is a fixed expression used adverbially for 'definitely, no doubt.' Both the subject of 大丈夫 and the 'thing' that will fit are omitted — context supplies them.