This is one of my concerns as well...especially the width and being able to fit 3 adults or 3 booster seats across the 2nd row. This would be a deal breaker in making this our family (of 4) car.
Last year, when Musk first started talking about the Model Y, he compared the size to a grand Cherokee. Which is what we currently use as a family vehicle and what I would like to replace in a few years with a Model Y. If it isn't as comfortable as the Grand Cherokee then unfortunately I will have to look elsewhere for a replacement and may have to give up on getting an EV...at least for now.
I guess we will just have to wait and see.