Hi,
It's normal to have a lower size of your browser when you use GetSize. Depending on your browser, the GetSize will return the area of your html render and not the size of your window. The browser has toolbars that eat the height of the page.
But the width is the most of the time the same than the one you requires.
Can you see your firefox running ? You will quickly understand why your GetSize is so low.
To set the size, you just need to write the property remote.Manage().Window.Size where remote is your remoteWebDriver.