The silverstone power supplies are a good quality unit, I run the 650W zeus and have not had an ounce of trouble with it and the thing is loaded up to the hilt.
As for size the 560W unit should be a standard size, I did find the 650W to be a little longer than some of the smaller wattage units.
I would take silverstone over Antec any day
Also have a look at this unit
http://www.pcpower.com/products/view...hp?show=S61EPS