For speedlight use, the Photoflex brand's formerly RUT model umbrella, aka the convertible model, is a good choice with Vivitar 285 or the Nikon SB 600 or SB 800 flash. I believe they have changed the specification away from the old three-letter RUT designation. 30 to 32 inchers work very well with speedlights.
You need to be careful of buying cheap, Chinese made light modifiers--many will color your light quite unpleasantly. it is well,well worth the money to buy name brand umbrellas; Westcott, Photoflex, Speedotron, Lastolite, and Photek all make QUALITTY umbrellas that deliver a pure, white light.
I prefer reflecting umbrellas over shoot-through ones because they control spill light much,much better. With a shoot-through, about half the light goes through the umbrella and toward the subject, and the other half bounces back and all over the shooting area, causing a lot of what is called ambient spill; with really powerful flashes, like 400 to 1200 watt-seconds, shoot-through umbrellas throw spill light all over the place. One advantage though is that the ambient spill can also act as a slight bit of fill light; in small camera rooms I have seen MWAC type family portrait shooters use one large shoot through umbrella, and in their low-ceilinged shooting areas, it provides main light AND fill light. Not that it looks professional, but it does create a low level of fill. if you want to simply bring up the light in a room, a big shoot-through with 200-800 watt seconds blasted through it will do that,especially if placed near a white wall, where the light going through goes one way, and the light blasting out the back side hits a reflective surface and adds to the overall light level in the room via bounce.
Using larger umbrellas, like 45 inch models, a speedlight really cannot disperse its beam wide enough to "fill" that large of an umbrella all that efficiently, but you can move the flash very close to a shoot-through umbrella, and get a soft central area of illumination with very dramatic fall off or feathering of the light; the speedlight will shoot through the very center quite nicely, and the edges will have a scant bit of light.