'The size is wrong.' or 'This is the wrong size.' would sound much more natural.
I prefer the translation as is, it is a more direct translation that helps me to learn. The more natural translation is not as helpful.
It should say "the size is not right' which matches the Korean and is correct English. Another correct sentence (if we know what item is being discussed, for example a shoe) would be, "The shoe does not fit." But it is not correct English to say, "the size does not fit." And also it's completely unnecessary, since 맞지않습니다 can be correctly translated as, "is not right." Also it would be ok to say "The size is wrong."