Get the custom sized images of authors

Hello everyone,

I am working with Wordpress where on home page i want to show the authors who has got most mosts in the blog, the author whose post has got most comments and like that most viewed author.
All these images i want to show in a slider with thumbnails.

Although, i have integrated a slider on the page and i am getting the images of the authors.
The thumbnail images which i am getting is in proportion (for which i am using get_avatar()) function).
The same function i am using to get the main image of the slider, which of course i want in much bigger size. But that image is getting distorted when i pass greater size vale in the get_avatar() function.

Can anyone suggest me nay other way to get the greater size image of the author…