Can i increase the height of the header menu slightly so that Logo can appear bigger?

To add a bigger logo above the nav bar, you can open up the header.php file in themes/dw-fixel folder, find the Line26:
Replace the following code

<?php dw_fixel_logo() ?>

With new code:

<div class="dw-logo"> <a href="#"><img src="<span style="color: #ff0000;">http://placehold.it/350x100</span>" /></a> </div>

Note: You can add your logo to the Media then copy FILE URL in the right hand side.
If you still face there issue, you can send me your site for further checking.
Hope this helps !