I have recently bought the 42cm frame and have a standover height of about 75mm and am 5'8" tall. These frames appear to be generously sized compared to some due to the more than adequate top tube (head tube to seat post) length. I would advise that the 42cm frame is fine for people of 5'6" to 5'10". 6 footers may prefer the 18" frame. A word of caution.....dont forget that due to the slope of the top tube (verrry sloped compared to many frames) you will need at least a 350mm seat post (26.8mm) but preferably a 400mm post to avoid over-stressing the seat tube top weld.
In response to Richard's question, I was talking of the XC frame. Mike's comment on seatpost length can be explained as the 19.5" frame has a similar top tube length to its smaller brothers (I think its only half an inch longer), but the frame size difference is achieved by using a taller seat tube, therefore a shorter seatpost length will result as Mike points out. The smaller frame does have a different geometry as a result (lower slung and a more sloping top tube) which I personally prefer, but the downside is you may have to use a longer seatpost (depending on inside leg length odf course). Whatever frame you decide on, they are all great and handle very well (I ride with three other people who all have Maxlight XC frames of different sizes).