How big of an umbrella?

How big of an umbrella is practical when using a 580 or 430 ex off camera on a light stand as the main or key light? In other words, even if I pull out the flash diffuser panel, can the flash cover the wide area of a large umbrella or is there just a big hot-spot in the middle?

Currently I have the Impact "kit" umbrellas which are smaill - I believe around 32". I can move them in close to the subject but was wondering if I would benefit from a larger umbrella.

Thanks for your advice.

Depends. The larger the area of the subject you want to cover evenly, the more important a larger umbrella is. You decide.

Assuming 28mm (FF) coverage setting on the flash, it covers 25" x 37" dimension at a distance of 30" from the umbrella material. At 50" away from the material, it would cover 42" x 63" dimensions. Of course, the lighted area is actually larger than the dimension stated (which is a lens FOV). Use a program like f/calc which computes lens FL coverage FOV to give you an idea of what area can be filled

The biggest I have used is 45". This works easily without the diffusion panel pulled down. I probably wouldn't go any bigger than this.

I use 43" umbrellas for my 580EX II flashes. Work great for everything I have tried them with so far.

From what I can tell Zack Arias over at OneLightWorkshop regularly uses a Vivatar 285 with a 60" so I don't think it would be a problem with either of the Canon units. As Wilt commented I don't know if it will fill the umbrella but it should be close, they won't fill a ceiling either but you still use bounce flash. As Frank said if the subject requires a larger light then go for it. For the most part an umbrella is a pretty cheap add on that just gives you an extra tool in the bag.