Auto Login & ResizeBy: nairol This Launcher MOD allows you to modify your screen size, auto login and allocate memory from the Options menu of the Minecraft Launcher. DOWNLOAD LINK INSTALLATION Open the file Contents/Resources/Java/minecraftlauncher.jar (must be the .jar version not the .exe version of the launcher.) Copy the contents of the AutoLoginAndResize2.zip /net/minecraft/ into your Contents/Resources/Java/minecraftlauncher/net/minecraft/ folder. Open Contents/Resources/Java/minecraftlauncher/META-INF/ and delete only MOJANG_C.SF AND MOJAG_C.DSA. DO NOT DELETE MANIFEST.MF. Repackage your /minecraftlauncher/ folder into minecraftlauncher.jar and replace the original.
They have a Windows .exe video, but I am not in the business of theft so you will need to check the original Minecraft Forum thread for that info. (Click the author name) However, this is the preliminary steps needed to modify the .exe version as well, so here is your head start.