Lots of sites poo-poo'ing this story, eg :
http://www.ww2aircraft.net/forum/aviation/anna-kreisling-26658.html
http://forum.axishistory.com/viewtopic.php?f=5&t=174537
And as for a comparison with the B-29:
Length - Ju390 112ft B-29 99ft
wingspan - 165ft 141ft
height - 23ft 29ft
max weight - 75500kg 61000kg
So bigger, but nowhere near twice the size.
And if you believe Wiki :
"
New York flight
The first public mention of an alleged flight of a Ju 390 to North America appeared in a letter published in the November 1955 issue of the British magazine RAF Flying Review, of which aviation writer William Green was an editor. The magazine's editors were skeptical of the claim, which asserted that two Ju 390s had made the flight, and that it included a one-hour stay over New York City. In March 1956, the Review published a letter from an RAF officer which claimed to clarify the account. Citing unspecified German aircraft records in his possession, the officer said that only one aircraft, the Ju 390V1 prototype, had made the flight in the latter part of 1944, and that it had reached a point about 19 km (12 mi) off the US east coast, just north of NYC, before returning to France.
According to Green's reporting, in June 1944, Allied Intelligence had learned from prisoner interrogations that a Ju 390 had been delivered in January 1944 to FAGr 5 (Fernaufklärungsgruppe 5), based at Mont-de-Marsan near Bordeaux, and that it had completed a 32-hour reconnaissance flight to within 19 km (12 mi) of the US coast, north of New York City.[1][9] This was, however, rejected just after the war by British authorities.[10] Aviation historian Dr. Kenneth P. Werrell states that the story of the flight originated in two British intelligence reports from August 1944 which were based in part on the interrogation of prisoners, and titled General Report on Aircraft Engines and Aircraft Equipment; the reports claimed that the Ju 390 had taken photographs of the coast of Long Island. These photos have never been discovered.[8]
The claimed flight was mentioned in many books following the RAF Flying Review account, including William Green's own respected Warplanes of the Second World War (1968) and Warplanes of the Third Reich (1970) but without ever citing reliable sources. Further authors then cited Green's books as their source for the claimed flight. Green himself told Kenneth P. Werrell many years later that he no longer placed much credence in the flight.[11]
Werrell himself later examined the available data regarding the Ju 390's range and concluded that although a great circle round trip from France to St. Johns, Newfoundland was possible, adding another 3,830 km (2,380 mi) for a round trip from St. Johns to Long Island made the flight "most unlikely".[12]
Karl Kössler and Günter Ott, in their book Die großen Dessauer: Junkers Ju 89, 90, 290, 390. Die Geschichte einer Flugzeugfamilie ("Great Dessauers...History of an Aircraft Family"), also examined the claimed flight, and thoroughly debunked the flight north of New York. Most importantly, it was nowhere near France at the time when the flight was supposed to have taken place. According to Hans Pancherz' logbook, the Ju 390V1 was brought to Prague on 26 November 1943. While there, it took part in a number of test flights, which continued until late March 1944. Secondly, they also assert that the Ju 390V1 prototype was unlikely to have been capable of taking off with the fuel load necessary for a flight of such duration due to strength concerns caused by its modified structure; it would have required a takeoff weight of 65 tonnes (72 tons), while the maximum takeoff weight during its trials had been 34 tonnes (38 tons). According to Kössler and Ott, the Ju 390V2 could not have made the US flight either, since they indicate that it was not completed before September/October 1944.[5]"
