I would guess that, in line with many publications, you can write whatever length you like, you would just have to be aware that you might get edited down for clarity. Mainly because I read some letters that I can't believe were written with the concise thought that appears in print, and fear that some of the more 'green crayon' thought might be excised for brevity.It seems to depend on the subject matter of the letter or the infamy of the correspondent: I've seen page long letters published.
So long as it keeps to the point, there's no limit per se but the longer it is the more likely it'll be edited in print. Keep it as succinct as you can.Could anybody please let me know the word count limit for letters submitted to Fortean Times? I can't find any information in the magazine.