changing header size of my theme

Hi everyone
I am new to wordpress and would really appreciate your help
I have been playing around with a theme called Interphase
I changed the logo image to my logo however the size is too small
I want to fill the whole header of the theme with my image. how would i change the header size so that my image sits nicely within the header
i have tried to increase the logo sized but it just increases the whole header and looks out of proportion. I have tried to change the size in the header.php file but I cannot seem to find in the source where I can change it
also this error message keeps coming now and will not show the theme correctly
Fatal error: Call to undefined function the_content_limit() in C:\wamp\www\wordpress\wp-content\themes\InterPhase\home.php on line 50

thanking you in advance