If it was that big, that would be the Nissan President, which is known here as the Infinity Q45. It's a big car, no doubt, and is used largely as a company car (those discreetly badged company limos) - as a privately owned car in Japan, it's definitely a niche vehicle. It's wide, but in fact, the MDX is wider.... I didn't say that the MDX is "too" wide, only that the width will be a problem. About driving in Japan (the streets are confusing - the drivers are generally pretty good though)........ Navi in Japan isn't a luxury, it's a necessity!
Fireblade: I didn't know that Toyota sold the Lexus brand in Japan. I've seen them in Singapore, but only Toyota in Japan. Maybe the ones you saw were re-imports (or Land Cruisers that were re-badged by the owners). I have to admit that I've only been to Japan on business (although many times), and don't spend a lot of time outside the city centers. The only Land Cruisers I've ever seen were around the airports, being used as Security vehicles.
The MDX's left hand drive's still a MAJOR limitation...........