Currently, Bp Gallery does not provide a way in admin to set the specific size for video/audio player. you can control it via this filter.
here is an example
[sourcecode language="php"]
add_filter("bp_get_gallery_media_sizes","change_media_size",10,2);//filter on the media size settings
function change_media_size($settings,$media_type){
if($media_type=="video"){//if video, change settings
$settings['thumb']=array('height'=>270,'width'=>480);//use in activity/thumb
$settings['mid']=array('height'=>270,'width'=>480);//mostly not used
$settings['larger']=array('height'=>360,'width'=>640);//used on single page
}
//for audio
else if($media_type=="audio"){
//change the settings array similarly as the video
}
return $settings;//return the size setting
}
[/sourcecode]
Hope it helps