"How big is it?"
Translation:Quanto è grande?
To my non-native ear, "Quanto grande è" almost demands another subject, as in "Quanto grande è la tua casa?" I can't recommend Tony628's way of thinking; it is more like "How much is it/he/she big?" or "To what degree is it/he/she big?" "È" includes a subject, and to ignore that is to misperceive the Italian form.
"Quant'è grande?" is indeed the most natural translation. "Quanto è grande?" is not wrong, but I think it is more natural in a context in which you want to emphasize the word "quanto". Suppose, e.g., that someone has asked you to carry or store a box for her and has repeatedly warned you that it is a big box. You might ask: "Sì, ma quanto è grande?" Finally, "Quanto grande è?" is the least idiomatic of the three. I agree that this word-order sounds more acceptable with an explicit subject ("Quanto grande è la tua casa?") I think I only heard this last form by itself (without explicit subject) in the North-East as an expression of surprise and marvel. E.g. someone is proudly showing you her very big house and you might go as if you were incredulous: "Ma quanto grande è?" (This would be taken as not requiring an answer).