You have to count the syllables. しゅう is twice as long as しゅ, but the vowel is the same. (An alternative way of writing the pronunciation would be シュー and シュ, which makes the vowel length more obvious.)
Similarly, せい is usually pronounced セー (as a long vowel エ).
Native speakers of English, for example, have a high risk of not being able to pick up on the length of a syllable. This fact is made worse when one considers that sometimes romanized Japanese is displayed (incorrectly so) without the macron, e.g. kenshusei, which should be kenshūsei.
Counting syllables is not only important for vowels, but also for "consonants" (namely ん) as in こんにちは, often mispronounced as konichiwa (no syllable only for "n").