Choose a random word from the whole list of 3-8 letter words?
Choose a length, then choose a random word of that length?
Should it be equally likely that you'll get a word of each length, or should it be weighted based on the actual number of words of each length?
I'm leaning toward making it equally likely that you will get a word of each length. Otherwise, you would just tend to go for 6-letter words more often, or whichever length there happens to be the most of.