"That bed is big."
There are many words in Japanese that can be expressed with the same hiragana. For this sentence, it is good to note that there is no komma after あのin the Japanese sentence. So it is not "um". In conversation you will also hear the difference: the "um" often trails off (like this 「あのー」or 「あのう」) or has a pause after it.
As for the difference between あの and それ.
There is a group of four kana with special meaning: こ、そ、あ、ど In combination with other kana they form a special group of words. The first kana describes the physical/psychological distance and the other marks the word as a pronoun, a location etc.
この = this ___ (the noun is near to the speaker)
その= that ___ (the noun is near to the listener)
あの = that ___ over there (the noun is far from both speaker and listener)
どの = which___? (used for questions)
これ = this one (the noun is near to the speaker)
それ = that one (the noun is near to the listener)
あれ = that one over there (the noun is far from both speaker and listener)
どれ = which one? (used for questions)
The form with -の is used to together with a noun. The form with -れ is used instead of a noun.
You can find a longer explaination here: http://www.japaneseprofessor.com/lessons/beginning/demonstratives-the-ko-so-a-do-series/
If you were given the English sentence, "that bed is big" and translated it as, "sono beddo wa(ha) ookii desu" (with appropriate hiragana and katakana), then your answer should be considered correct. If it wasn't, you can click the flag to report it, and when course contributors have a chance they will add it as a correct translation.
I think this changes the sentence structure completely. This is more like "that big bed over there" or something like that. Instead you want to say, "that bed over there is big"
If you think about it like english youd say "that bed is" = "あのベドは" and then you can follow thst with whatever description you want. In this case, it is big so おきいです。