1. Rendering Size feature sets the rendering size of the game window, assuming you pick a size. If you type in 840x525, it's going to put them all, including your main window, at 840x525 (the main window would scale UP the display, as opposed to the other 4 scaling DOWN).
2. You can remove the border from the WoW window using in-game resolution options. Windowed mode + Maximize + Disable Resize. But, that option should work. You dont have an older windowsnapper installed do you? Like from Zanthor's original posts?
3. They're "unclear" because they're 25% of the original size, and still rendering at the full size. The display is scaled down.

It is possible to have 4 of them rendering at 840x525 and one at 1680x1050, but not through the current options in ISBoxer. However, if you were to do this, mouse repeat will be inaccurate -- if you don't need that feature, then perfectly viable. If you didn't need them to swap to the main area, then you could of course do it without any extra work, just by disabling the Swap option. The problem otherwise is that you're wanting it to switch resolutions to 1680x1050 in the main area, and this is going to take a few seconds instead of being instant -- this is why we keep the resolution identical in all windows.

I don't know where the 800x600 number comes from unless someone was just going down the list of resolutions in the resolution options in the game. It's not a hard limitation.