Is there a significant advantage of having a 17" screen over a 15" or a 19" over a 17" or is 15" pretty much optimum? 17" & 19" can natively display 720p, the minimum resolution required for High Definition. Both of these usually only run 1280x1024 so no increase in resolution is gained by going to a 19" from a 17" Theoretically, the 19" can gather more light from your source so you may achieve a brighter picture by using one. Increasing the LCD size gives you a shorter throw distance for the same size projection and the casing size will obviously increase with the larger LCD. The 19" (and a 17" using the Pro Lens set) will require a longer box as well, due to the longer focal length of the Pro Lens set. 15" screens are getting rarer these days, but are a tried and true option for the smallest box and a very acceptable picture if you're not chasing true high definition.

The highest definition LCDs right now are actually smaller, 15.4" being fairly common for something that will do 1920X1200, which can display 1080p high definition.