That would either be 50" or 60". Most brands only offer 50" or 60" (ie LG, Pioneer, Panasonic do 50"). Samsung offer 58" models. How much are you wanting to spend? And what are you going to be using it for?
...could you stretch to a 60"?
Panasonic will be releasing the P54Z1 soon which should be mighty impressive, but it will cost you an arm, a leg, a kidney and your house, maybe even through your car in for good luck! Otherwise they only offer up to 50" unless you go with their professional range, which do come in other sizes such as 58"...again, they will cost you though
I'm not sure if this is a reliable source but was the first thing google pulled up when I searched - Panasonic's 2009 HDTV Range | Giant Buyer it also mentions a P54G10 though, which hasn't arrived yet to my knowledge (it says it will arrive in may)
The Panasonic UK range is different to other markets. There are no plans to bring the 54" screen to the UK other than with the Z1 which has a (Panasonic predicted) RRP of £4300 for the 46" and £5500 for the 54".
Later in the year we will be seeing the 58" and 65" V10, although no word on pricing yet.