This is really starting to annoy me.
I have tried using FTB Retro Pack because, for whatever reason, my computer will not run Tekkit Classic and FTB Retro looks as similar to it as I want.
When I launch FTB Retro a small window opens which then fits to the size of the screen (it does not enter full screen just a screen sized window.
A few seconds later the window goes back to being small before once again making itself screen sized, this is when the Mojang logo appears, but only in the bottom left hand corner in a size equivalent to the smaller window, when the Minecraft starting screen loads all I can see is the bottom left hand corner of it as shown by the screenshot.
Unfortunately whilst trying to see if I could fix it by roughly guessing at the options within Minecraft which I was able to see some of, I made the GUI really small and now can't see anything of any use.