CSS to change SP2013 pubic web site

I am trying to use CSS to modify my Public web site pages on new sharepoint 2013.    I was trying to increase the display size of my logo/siteicon in the header of my web pages.    I tried the following code:

.ms-siteicon-img{

height: 200px;

width: 200px;

}

But this is not working.   The maximum height  can enter is 70px,  after that the display size/height of my logo is not made any bigger.  I can increase the width of the logo/siteicon which does work. (but this just stretches the logo, I need to change both height and width by same amount so image still looks good)  I am using the "OSLO" look and feel and it seems like something else is limiting the height of the logo/siteicon to 70px.   Anyway to do this?

Thanks,

Mark