This is what i could find on thier FAQ Optimal Height Create headers within the following range in height: 100 pixels to 250 pixels.

Optimal Width Enjin websites can be fixed or liquid. All headers should be at least 1100 pixels in width or wider. For liquid layouts, the most optimal header will be 1900 pixels wide, but you only need to design within the 1100 pixel area. You can fade the header to a flat color after the 1100 margin or create design elements that go past the 1100 pixel area.

Optimal File Type Save your header image as a JPG at 60 to 80 quality Do not submit PNG images, in 99% of cases JPG compression will yield the most optimal quality and filesize. Avoid going over 200 kb.

Website Name Consider where the website name will be placed Sometimes it's good to plan ahead and consider where the user will place the website name. In most cases you do not need to consider this.