this 이 much 만큰 ... Think about seeing two beds. With this sentence you point at one that is near the same size that you want, or have.

+1

Because 1: lesson is asking for English words.

이만큼 means "this much", but gives no indication of -what- it is this much of. You need to use 크다 [큽니다/커요] to indicate that "bigness" is what you're talking about (instead of "expensiveness" - 이만큼 비쌉니다, or tastiness - "이만큼 맛있습니다", etc.)