Logo size and portfolio picture size

Hi Team I am wondering the maximum size (in pixel) of the logo in Audioman Pro theme? Can you please take a quick look of my current logo on www.reservehifi.com.au ? I am happy with the current size (1280x1280) when viewing from a desktop. However, the logo is too big on mobile phone. What is the solution here? Furthermore, I am seeking solution to the portfolio section of my website. I like the 5 columns layout. But 4 portfolio pictures are not the prescribed size 640 x 640. Corping or resizing the picture would make it strange. Is there another solution? I am looking forward to your reply. Kind regards Ben