This themes provide option to change logo & favicon. You will also need to prepare your retina logo & favicon.
Before changing favicon, you should create 2 fav icon, first for normal display, and second for retina display monitor After that you can easily change favicon from Customizer»General options»Favicon
Default size of favicon is 16×16 Pixel, and 32×32 Pixel for retina display favicon.
You can change logo from Customizer»Header & Navigation »Logo & Logo Retina. if you are using Menu position Side. You will also need to fill Side menu logo that located on Customizer»Header & Navigation»Side Menu Logo & Side Menu Retina Logo
There is no limitation for size of logo, but please consider responsiveness of your website when creating & inserting logo.
You can also change padding of your logo for both default Top logo & Side Logo. You can find it on Customizer»Header & Navigation»Logo Top Padding,Logo Bottom Padding,Side Menu: Logo Top Padding,Side Menu: Logo Bottom Padding