Size of Time Machine drive?

Hi. I have been running a 500MB external as a TM backup to my 160GB MBP. It never complained of being full and I've run it for about three years. What is the recommended size of an external for this job in relation to the laptop's hard drive size? I have just changed my MBP drive to a 750GB so am sure I now need a larger external for the TM, is their a rule of thumb for this does anyone know?
Jules

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