I’m running a Humminbird 788 on the bow of my boat. It has a 5″ screen (640 x 640 pixels), and is quite readable, even in split-screen mode. It will fit within your budget as well.
The 788 comes complete with Humminbird’s high-precision 50-channel GPS receiver (GR50), and is map-chip capable. In the immediate future, Humminbirds will be Lakemaster compatible (this fall). If you purchase a 788 now, you’ll be able to update the software with an easy, free update from the Humminbird website to make your current model Lakemaster compatible as well.
Picking up a brand new combo with a larger screen will be a challenge, unless you’re willing to get a lot closer to $1000 on the price tag. Used large-screen units are regularly available, and some of these will fit in your price range, but don’t forget to consider warranty issues when purchasing a used unit.