If you don’t get the right size your eyes or mouth may be missing from your Twitter cover photo. Although Twitter does help resize your image, it doesn’t always get it right. The correct dimension and size for your Twitter avatar is 73 by 73.
To crop or resize your photo to the dimensions you want it, it is best to use an image editing tool, to get the dimension you want for your photo. You don’t always need to have the exact size, try uploading your avatar pick and check to see if Twitter resizes it correctly for you.
The new Twitter cover design does not affect your avatar size, although it looks like the Facebook Timeline with your Avatar in the middle. You can add a cover photo as you would with Facebook; look at this Twitter page below for some ideas of what you can do with it.
The size for the cover header is shown above, you go into your Twitter settings>Design>Header^^