I not sure if this is supported but this is what I have done in the past.
I have either created or found larger icon's for the social media images that I want larger, mainly Facebook and Twitter. Google Facebook or Twitter icons; you will find plenty out there and make sure you save them as a .png file type.
I then use the social media extension and place it on my page where I want it.
I then go to the social folder created by the extension. This will be the %root%/webassist/social/icons and replace the original image files with those of your choice.
You will then need to edit the wasocial css file and change the width and height of the respective Facebook / Twitter buttons.
From memory they are commented so it will not be difficult to find and change these parameters.
Hope that this helps