” Yes, or you could say "His room is half the size of mine". In informal speech and writing, native speakers often use expressions such as "twice as small" to mean "half as large", so in that sense it's not true that you "can't say it". However, even though such expressions are grammatically correct, they are logically dubious.
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Alex+And we can’t say: “His room is twice as small as my room.” Instead of this we say: “His room is half as large as my room.”
Alex+I’m slightly confused because in my language it’s possible to say :”Road A is three times longer than road B.” or “Road B is three times shorter than road A.” That’s why it is difficult for me to understand than in English we can say “Road B is shorter than road A.” but we can’t say how many times