We have come to a question about a 6 foot long creature (FCE Star, p.76 l. 23). Typically you would say 6 feet, but here this is also correct, and actually more often used in this situation.

The rule could go like that: if we can see an adjective in the meaning (a four-year old boy - čtyřletý chlapec), we use the singular and we can also use the hyphen (-), whreas when it is not an adjective (the boy is three years old), we use the plural and no hyphen. Thus, we have a six foot long monster (šestimetrová) but a monster that is six feet long ()dlouhá 6 metrů.

Let's see what information we learn about that from good English information sources:

We get a reference to the

Anglo-Saxon (Old English) genitive plural

Why do we say "30 years old", but "a 30-year-old man"?
This pattern goes all the way back to Old English (alias Anglo-Saxon). It's the same reason many of us say that someone is "5 foot 2" rather than "5 feet 2".
The source of the idiom is the old genitive plural, which did not end in -s, and did not contain a high front vowel to trigger umlaut ("foot" vs "feet"). When the ending was lost because of regular phonetic developments, the pattern remained the same, and it now seemed that the singular rather than the plural was in use.