If you've uploaded a logo and it's appearing smaller than you expected it to be, this is because the theme downsamples the logo size by 50% to ensure the logo remains very sharp on Retina devices.
e.g.
If you upload a logo of 300px * 200px - that will actually render as a 150px * 100px logo on your website.
So, the simple rule of thumb is to upload a logo 2x the size what you want it to appear as.
So if you wanted to have a logo appear at 250px * 300px, you would need to upload a logo of 500px * 600px and then adjust the max header height to be at least 250px.