Lánjīng shì dìqiú shàng zuì dà de dòngwù, tǐcháng kě dá sānshí mǐ.
The blue whale is the largest animal on earth, reaching up to thirty meters in length.
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体长 is the written, scientific word of encyclopedias and field guides, usually for animals. In conversation people just ask 有多长 'how long is it', and a person's height is 身高.
