Yes, it always fit. But how about get current screen resolution and set window width and height proportionally and place window centered?
I don't see how it matters. as soon as you restart you get the size and location you had before..
And there is an idea to how windows is handling it. Because if you open a second instance you will not get the exact same position, it will be be moved a bit more down and to the right, of each new instance you open.. so you will se all windows of all instances. on top of each other but every windows is offset a bit so you see parts of all. seeing that you opened x instances and not one.
But you can do it with script..
MC.CmdLineRun CMD=":setpos 200 200 1400 1000"
Then from your MultiCommander shortcut you can tell it to auto run a command.. with multicommander.exe -AutoRun=<Name of Command to run>