To NY traveler: It is correct. I live here. You can gauge it by your cars odometer. If you go from say 4000 no. to 4800 no. it is 1 mile. End of story.

Ryan, actually in some places the island is almost 3 miles wide.

That is true, Go Travel. But that doesn't change the fact that it is two miles from 1st Avenue to 10th Avenue with 13 Avenues in between. Nor does it change the fact that it is roughly a 1 1/2 from the UN to Times Square.

Just to correct wantsomesun, the correct dimensions of Manhattan are 13 miles long and 2 miles wide, total area is 26 square miles.
The other information you've gotten is correct, 20 blocks to the mile, avenues (East/West) are generally the equivalent of 2-3 of the North/South blocks, the longest block in Manhattan is the block between 5th and 6th Avenues.

Thank you all for your replies, I can now carry on with planning our trip.