full length lighting with umbrellas

i assume that a larger (48" or more) umbrella or softbox is needed for full length photography...

how big of a modifier can one use with speedlites?

would it be feasible to position two speedlites with smaller umbrellas vertically in line and fire them both simultaneously for a similar effect?

I use a speedlight in a 60inch bounce umbrella all the time.

I use my 43" umbrella for full lengths often. It works great because the light feathers off by the knees.

I've used both umbrellas and a 150cm Octobox for full length portraits. If I'm in a hurry and want to travel light I grab one of my YN560-II speedlights with an umbrella. But the YN560-II has no trouble with being under powered when I do want to use it in the big Octo when required.

Thanks everyone...

i was afraid there wouldnt be enough power to really be able to utilize the size of a big reflector like that...

might have to spring for one and give it a go. i really want to shoot some full length stuff. or at least TRY to shoot it...

Here is a LINK by a fellow member, TMR Design that dealt with your question about whether or not one could use a speedlight in different sized modifiers effectively. As long as your speedlight supports wide angle zoom (24mm), you are good to go with very little difference noted by Robert.