Using an FTP client like Filezilla connect to your server then go to wp-content/plugins/ and look for the bp-custom.php file, if the file doesn't exist there, create it.
Path to the file : wp-content/plugins/bp-custom.php
To increase the avatar size make sure that you have next lines added to the file.
COPY CODE
if ( !defined( 'BP_AVATAR_THUMB_WIDTH' ) )
define( 'BP_AVATAR_THUMB_WIDTH', 120 ); //change this with your desired thumb width
if ( !defined( 'BP_AVATAR_THUMB_HEIGHT' ) )
define( 'BP_AVATAR_THUMB_HEIGHT', 120 ); //change this with your desired thumb height
if ( !defined( 'BP_AVATAR_FULL_WIDTH' ) )
define( 'BP_AVATAR_FULL_WIDTH', 580 ); //change this with your desired full size,weel I changed it to 260
if ( !defined( 'BP_AVATAR_FULL_HEIGHT' ) )
define( 'BP_AVATAR_FULL_HEIGHT', 580 ); //change this to default height for full avatar
Values 120 and 580 represents pixels.
For the avatar thumbnail avatar simply replace 120 with your desired values
For the full image avatar simply replace 580 with your desired values
