Ideal size for header images

Hi there,
Am trying to add a few header images so they appear in randomized order.

According to the Extra Theme customization options, it states:

“header Image (Deprecated)
Click “Add new image” to upload an image file from your computer. Your theme works best with an image with a header size of 300 × 100 pixels — you’ll be able to crop your image once you upload it for a perfect fit.”

What is the ideal image size that would need to be uploaded as 300 x 100 is definitely too small.Am I missing something here?

THanks in advance

Shalini