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."
"this size does not fit me." versus the above. And here we go again with the pronouns. The, this, these, or those" which one should we use, which one should we use? Just write those four words on separate pieces of paper and tape them to a wall ... then throw a dart and use which ever pronoun your dart sticks to. However, even after this you have only a 25 percent chance of your dart sticking to the correct piece of paper. You've got to figure this out Durolingo ... or give the students a pass on pronoun usage.