The OMF Administration panel* allows you to make some quick customizations, including this one.
*Note: The OMF Administration panel isn't included in OMF Lite. You can see a full comparison between OMF Lite and OMF Basic here.
OMF v1.4.x and later
- Go to the Extensions > Modules section of your OpenCart Admin Panel.
- Locate the Opencart Mobile Framework Administration and choose Edit.
- Navigate to the Customization tab. You will see this screen:
Specify a mobile logo first. Then you can control its size with the second setting on this tab—Mobile Logo Size. It's computed as percentage of the header size. You can just try out different values to see what percentage gives you the desired results.
Read more about the OMF Administration panel here.