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That's probably a good idea. A 5 pound circulating coin would probably be cool.

very sensible ... altho the one pound coin is kinda chunky ... wonder what a five pound coin would be like (hrm. bimetallic, like the two pound coin, but maybe with different metals ... 0

Most likely, any circulating £5 coin would have the same specs as the current commemorative £5 coins (which are all legal tender, but not widely accepted). They are all "crown" sized, 38.61mm, 28.28g, CUNI.

Incidentally, the "crown" coins issued prior to 1990 (which include 1972 Queen's wedding anniversary, 1977 Queen's Jubilee, 1980 Royal Mother's birthday, and 1981 Charles & Diana wedding), are all legal tender for 25 pence. The pre-decimal crown was worth 5 shillings (¼ of a pound), so when the conversion to decimalization took place in 1971, the crowns were automatically re-valued as ¼ of a pound, or 25p. (production costs made it prohibitive to continue this practice, so beginning in 1990, the crowns were given a value of £5)

Given the past figures on the levels of counterfeited 1 pound coins, I find the idea of 5 pounds coins a total joke. Either it is going to be minted with some levels of silver like the current Euro coins with more security features never seen before or we'll let the counterfeiters flood the market with counterfeit coins.