Do you know if the Lib Dems have specified their preferred size of constituency? Your '4 to 6' seems a fairly typical number, and I agree it gets unwieldy much above that. The bigger it is, the more proportional it is, and thus also the more chance of an extremist like the BNP getting in. I suspect 5 seats per constituency would restrict them to only 1 or 2 areas where they are (unaccountably) popular.
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Size of multi-member constituencies