I have reflectors from different brands and they are all quite the same (so the brand doesn't really matter I guess).
Lastolite has some with a quie unique design, so you might want to check them out (The TriGrip and the TriFlector are well known but they are quite expensive, too).
For the reflector I would suggest to buy a 5 or even 7 in one. Our 5 in 1 can be used as diffusor and has a black / white and silver / gold coating.
The size depends on what you want to shoot. Acutally we have several different sizes. A small one which is usefull for close ups, a medium sized one I use often in our home studio and a really big one (1,50m in diameter) which is really handy for full lenght portraits or if you need a diffusor for a couple i.e. a bride and groom.
A problem with the big one: It's quite unhandy, especially when there is some wind....
Edit: Oh and one last thing. There also different kinds of shapes on the market. The round one's might be the most commen but something like
this is especially hand when doing full lenght portraits. And you can use it as a quick background, too.