Newby Question on Softboxes and camera flashes.

Hi,

I'm looking at shooting with an off camera flash unit out in the open. I have some cheapo wireless transmitters but would like to soften the light coming from the flash and am considering a softbox.

i was wondering - how large a unit can you get that will work with a standard flash (that mounts on a camera)? (Something like the Metz 48 AF-1 flash for instance.

I've seen some small ones on ebay 15cm / 6" but do they do much in the way of softening light, or am I better off just holding a diffuser the other side?

I've seen some larger ones 50cmx70cm / 20"x28" but do they work with these sorts of flashes, or do I need studio sized strobes for their size?

Thanks in advance.

Adza