For the most part, avatars on Tumblr are displayed at the same size and are used to indicate which user has posted an item. Tumblr displays avatars next to posts and replies to your posts as well as in your ask box to help you distinguish the post author.
Uploading an Avatar
Tumblr does not impose any limitations on the photo that you upload as your avatar. All uploaded photos are resized to 64 pixels high by 64 pixels wide. If your photo is a rectangle, Tumblr will crop the sides to make it square. Pictures smaller than 64 pixels by 64 pixels will be expanded until they fit; this may result in some cropping of the image if it needs to be expanded more one way than the other. For example, an image 25 pixels high by 50 pixels wide will expand the height so far that the width is lost.
Your avatar will always be the same size on the Tumblr dashboard: 64 pixels by 64 pixels. A user's avatar is displayed in the upper left corner of her post; if she has several posts in a row on your dashboard, it will be displayed only on the first post. Your avatar is the same size on your Tumblog -- the list of your posts and notifications displayed in the dashboard format.
When a user reacts to one of your posts -- either with a like, a reply, or a reblog -- you'll get a notification on your dashboard and on your Tumblog along with other posts. The avatar used on these posts is smaller than on regular posts, coming in at 40 pixels high by 40 pixels wide.
When you receive a question via Tumblr's message system, the user's avatar will be displayed beside their question. As with your dashboard, this avatar is 64 pixels by 64 square. A username will also be attached to the question.