Today an observant Tesla owner was lucky enough to be driving directly behind a Model Y prototype while it drove past another Model 3. This gives us the first side by side view that I have seen from the back of these 2 vehicles. The video can be seen in the link below:

Since the Model 3 dimensions are well known, I took a screenshot of the two vehicles while they were side by side (as best as I could determine) and was able to put the following dimensions together for the Model Y based on the pixels per inch of the Model 3 to the right. I did not measure the image for the Model 3 width since the camera angle is skewed, but I did measure the height of the Model 3 to 207 pixels. The actual height of the Model 3 is 57 inches, which gives us 3.754 pixels per inch in this image below.

Using this calculated result of 3.754 pixels per inch, we can apply the pixels of the Model Y height, which are 234 and calculate that the result is 62 inches in height. Using the same logic, the width calculates to 74.5 inches.

The final result shows us that my rough estimate is that the Model Y is 5 inches (127 mm) taller and 1.5 inches (38.1 mm) wider than the Model 3.