Z-M has given you the important link.
You don't need to worry about the size of the file you use (or the initial shape of the image). When you go to the Gravatar website you can either upload an image from your computer or provide a link to the location of an image on the web. You'll then be provided with a cropping tool which will get it into the required square shape. If you're asked to assign a 'rating' to the image, select 'G'. (Ratings are based upon the US film classification system with 'G', for 'General', being the equivalent of our 'U', for 'Universal'). The file size is automatically reduced to meet the needs of Gravatar's system.
You can have as many images as you like stored on the Gravatar website. You simply click on the one you want to be your current one. (e.g. the Christmas avatars I've used in the past are still stored on Gravatar. In December I can simply click on one of them to bring it into use, without the need for any fresh uploading or cropping).