The Japanese are going head-to-head with heavyweights New Zealand and South Africa to stage the event - and also become the first Asian nation to host the showpiece tournament.
The centre-piece Yokohama Stadium - which hosted the 2002 FIFA World Cup final between Brazil and Germany, and was also the venue for the Republic of Ireland's 3-0 Group E win over Saudi Arabia - holds a capacity of 70,000 and is located in Japan's second largest city.
Millar - while touring Japan ahead of November's decision on the 2011 host nation - commented: "I think it's (Yokohama) a very, very excellent stadium and it would be certainly worthy of the World Cup final.
"But Japan's stadia are both good enough and big enough certainly."
With New Zealand - the venue of the inaugural RWC in 1987 - yet to receive backing from their own government, it looks increasingly like a two-horse race for the 2011 honours.