Set maximum upload size for avatars
It is recommended that you limit the size of the images that your members are able to upload onto your website. In this way you can prevent your users from wasting disk space on your server.
In order to set a size limit, add the following line to your theme's functions.php file, or to your wp-config.php file.
define( 'WPUM_MAX_AVATAR_SIZE', 10240 );
The above example will set the maximum file size for avatars to 10kb. The WPUM_MAX_AVATAR_SIZE constant accepts only bytes as a value.
50kb = 51200
100kb = 102400
1mb = 1048576
You can use a website like this to help you calculate the any size you want http://www.whatsabyte.com/P1/byteconverter.htm