Extension methods to save and load form size, position, and state
Unnecessary code

The windowState variable isn't used in the SaveFormSizeAndLocation method. And I believe you don't need such variable at all, because form.WindowState.ToString() returns exactly the same string.



Suggestions

The SaveFormSizeAndLocation method.
You check the key for null value, and if it is null nothing happens. Maybe it makes sense to omit that check and to handle (or raise) the exception in order to inform user or to save the reason in the application log.

The LoadFormSizeAndLocation method.
You can reduce nesting by inverting the condition:

using (RegistryKey key = Application.UserAppDataRegistry.OpenSubKey(form.Name))
{
    if (key == null)
    {
        return;
    }

    form.Left = (int)key.GetValue("Left",form.Left);
    form.Top = (int)key.GetValue("Top",form.Top);
    ...
}




Naming

I believe that extension methods for the Form class shouldn't contain the word 'Form' in their names. Consider to shorten names: SaveSizeAndLocation and LoadSizeAndLocation.



Styling

It looks unnatural for me:


Spaces after ( and before ). I'd prefer to insert spaces before the ( in operators: 

if (x != y)


and to omit spaces in method declarations and calls: 

someMethod(int a, float b, string c);
someMethod(a, b, c);

No spaces between method arguments. I'd prefer to add a space after each comma:

SomeMethod(firstArg, secondArg, thirdArg);


This should improve readability.
Personally, I cannot easily distinguish between

SomeMethod(abc,def,ghi); // 3 arguments


and

SomeMethod(abc.def.ghi); // 1 argument