in file : /modules/boonex/forum/integrations/base/callback.php
Find this line:
$aRet['avatar'] = BX_AVA_URL_USER_AVATARS . $aProfileInfo['Avatar'] . 'i' . BX_AVA_EXT;
And remove the . 'i'
$aRet['avatar'] = BX_AVA_URL_USER_AVATARS . $aProfileInfo['Avatar'] . BX_AVA_EXT;
And it will use the bigger avatar thumbnail instead of the small one.
Don't forgot to tweak CSS and the AVATAR size in file:
/modules/boonex/forum/layout/base/xsl/avatar.xsl