1000.4 is the normal Eastern Arc dish desihned for 80% of the installs
1000.5 is the dish for lower signal areas
As for the size 1000.4 will be close to the 1k.2 the 1000.5 will be a little bigger. As for the exact specs I dont know.
The 1000.4 dish will be equal in size to the 500+ dish. In fact, the mast used for the 118.7 dish (500+) is now labeled as being for the 500+/1000+/1000.4/Border Dish. I assume Border Dish to be the 1000.5.
Also, the reason for a larger dish for EA is because 72.5 and 77 are so close together, that the LNBs will be picking up unwanted signals from nearby satellites. These satellites are normally 9 degrees apart, but in EA, they will only be 4.5 degrees apart. That's a lot of interference so a larger dish will increase the gain on the desired satellite at their respective LNBs, thus creating a better signal to noise ratio. Think of it like being in a crowded, noisy room and you're trying to listen to one person. If everyone is fairly distant from the person you are listening to, its easier to hear them. However, if all those noisy people are nearer to the person you are listening to, its that much harder to hear them. Larger ears would help that.
The spacing between 61.5 and 72.5 is 11 degrees. I'm not 100% certain a 500 dish would get both of those that well at the same time. Just depends if you can be 1 degree off of the satellite and still get it. A 500+ might work better for that and then just use the 300 for catching the 77 satellite.