I bought a 5in1 22" (56cm) reflector off eBay and it's done me good so far. Cost me less than A$30 landed! If i were to be getting a round one now, i'd probably go for a 32" (82cm)!
Also, as mentioned, you may wish to look into a rectangle shaped reflector, good for portaits or groups (reflector held in a landscape perspective)
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Canon EOS 400D w/ Battery Grip
EF 17-40mm f/4 L | EF 24-105mm f/4 L | EF 50mm f/1.8 II | EF 75-300mm f/4-5.6 | EF-S 18-55mm f/3.5-5.6 | 2x 580EX II
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