If you want something more extensive than Diceware's wordlist, check out
http://www.poslarchive.com/math/scrabble/lists/I copied the lists of 5, 6, 7 and 8 letter words into a spreadsheet. Used the lists "Common to TWL and SOWPODS."
Then I used random.org to generate random numbers to choose words from the spreadsheet.
Came up with some real obscure words. Kind of a trip looking up those oddball words and finding out what they mean.
Anyway, you can calculate the bits of entropy knowing how many words total you are choosing from.
The bits of entropy is the number of words in your string times the log (base 2) of the number of words in the master selection list (78521 -- somebody check me on this). So if you use six words you will get about 97.5 bits of entropy. This assumes you choose randomly from the master list, with each word having an equal chance of getting chosen, and you
don't use your own judgment -- no dropping a word you don't like or any such fudging.
Putting spaces between them will probably increase the entropy a little bit, I'm not sure how to calculate that.
