Short Answer: Unfortunately the User Profile Plugin can not affect the size of the profile pictures.
Detailed Answer: Larger profile pictures are a highly wanted feature request for Confluence - see also CONF-9375. They would make Confluence more social: Users will recognize other users in the system just by their profile picture.
The User Profile Plugin uses the profile pictures that are provided by Confluence. During the upload of a profile picture Confluence limits its size to 48x48 pixels. Due to that limitation we can not use larger profile pictures in the User Profile Plugin - although we would like to! (Stretching the existing pictures is not an option - they will become fuzzy and grainy)