On April 27th 2019, Teslarati.com release an article depicting the to be released Model Y sitting outside the Fremont factory Link Here.
One picture in particular shows a rear facing angle view, which allowed me to play around with the image in my favorite photo editing software and take each of the 19 inch sport wheels and place them side-by-side to guestimate how long the rear cargo area is on the car.
Original Image (Courtesy of teslarati.com):
Edited Image showing rear cargo area length, using the 19″ diameter rim size as a measuring guide:
Basing the starting measurement at the ridge seam on the roof and ending the measurement at the tailgate seam for the tail lights, it appears to perfectly fit 2 19″ sport rims. So using this as a rough estimate of the front to back length of the storage area, that would be 38″ long.
The reason I started the measurement from that spot is because in the below rear facing image, it seems as if the second row seating ends right around the start of the ridge seam. (Courtesy of teslarati.com)
In comparison, below is the inside measurement taken in my 2013 Honda Pilot from the back of the rear seat to the seam of the tail light which shows 48″ in length.
So using my “guestimation”, the rear of the Model Y cargo area is roughly 39″ long which is not the largest storage area, considering the roof slopes down considerably toward the rear of the car, but should be a good size when compared to other small size crossover cars.