I just tried data-size="large" and it worked. The fb_button_large style was applied to it, and it was a bit bigger. It was not much more, although I hardly noticed. In addition, it is not documented, so I assume that it will depreciate in the end?
data-size="xlarge" was GREATLY bigger, however.
<div class="fb-login-button" data-show-faces="false" data-max-rows="1" data-size="xlarge">Login with Facebook</div>
So I'm not sure why it is not working for you. Instead, I would use CSS if you want to customize the button completely.
The new button is just a nested construct of <div class="fb-login-button"><a class="fb_button"><span class="fb_button_text"> DOM, and you can use CSS to style it <div class="fb-login-button"><a class="fb_button"><span class="fb_button_text"> you want.
If you have a cascading priority issue when using Facebook styles, simply use !important at the end of the style declaration to override Facebook tags.
You can even resize the image to "F", as this is a sprite with multiple sizes . When you redefine styles, just play with the background-position property until you get the correct one.
Good luck
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