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

 





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.