"He writes a long letter."
Translation:Il écrit une longue lettre.
Apparently long / longue / longs / longues can either precede or follow the noun without changing the meaning.
There is quite a detailed discussion (between English and French speakers) here (in English) :
The placement of most adjectives in French is after the noun. There is a small group of adjectives, however, that normally precede the noun. These adjectives may be categorised as adjectives of Beauty, Age, Numbers Goodness, and Size (BANGS). longue describes length so it fits into the last of these - Size.
This is from a French poster on the WordRef forum:
For size/length/etc, it is usually placed before the noun:
"une grande maison". "un long chemin".
However, if you're using several adjectives, they are usually placed after the noun
"une maison grande et fonctionnelle". "un chemin long et douloureux".
This latter point is because the two adjectives differ - one belongs to BANGS and the other does not. However, if both adjectives belong to BANGS, they are placed in that order before the noun
"une belle, vieille maison".