Im looking to get a couple 150cm octaganol softboxes (maybe bigger). The umbrella style ones that pop up and mount to a bowens connector. I do a bit of on site shooting and my current rectangular ones which are put together ones just take too long to put up. (its all relative....). I do portrait work and use 300 what studio flashes. My question is; is there a correlation between softbox / umbrella size and the power of your flash? I would think at some point there would be but Im not trying to flash the inside of a barn. Can anyone point me to a reference on this or give me some personal experiences?