When you upload your logo image, please consider that it is used for retina displays and you should upload a logo double its usual size.

Otherwise, it will appear small on non - retina displays.


For example: 

if you want your logo to be 250px width, you have to upload a 500px width logo

Please note that both "Custom Logo Picture" and "Alternative Logo" must be the same size.


Also, the logo occupies the header area and regardless of its size will fit this area. 

You have to change the Header height through Dashboard > Appearance > Customize > Header > Header Height in order to make it appear bigger.