When it comes to casting, there are all sorts of things that producers and directors are looking for in their actors, which means there are numerous reasons an actor might not get a part. Many of them the actors have absolutely no control over. Zendaya was afraid that one of these issues might have cropped up when she auditioned for the role of Michelle in Spider-Man: Homecoming. As it turns out, she's a lot taller than her co-star Tom Holland. According to Zendaya...
As ridiculous as it sounds, actor height can be a significant image in the movies. Most of the time the movie wants to show their male protagonist as being powerful and imposing, and so when it comes to visual storytelling, that means making him the tallest person in the frame. Quite often, if the actor isn't tall enough to pull that off, it means filming in such a way as to create the illusion that he is. Zendays tells Variety that she clearly saw her height as a problem, as the movie would likely want Peter Parker to be taller than the women in the shot.
While sitting during the audition probably helped the initial visual in front of the director, it's unlikely that her height wasn't taken into consideration at all. Looking back, we're realizing that a lot of the scenes that Tom Holland and Zendaya share either have them sitting, or not standing very close to each other. It's possible this was done intentionally in order to hide the height difference.
To be fair, the height difference between the two isn't that great. Zendaya is 5' 10" and Tom Holland is 5 '8". Zendaya is absolutely tall for a woman, but it's not like Holland is particularly short. It shouldn't be too difficult to make Holland look taller on screen if that's the goal.
Of course, it's also possible that the extra height worked in Zendaya's favor. If you want to make Peter Parker look like an unassuming nerd, making him look shorter than everybody else would be one way to do that. In that case, a tall woman is what you want. People wouldn't expect Zendaya to be 5' 10", which means Tom Holland looks short when standing with her.
In the end, whether the height was important or not, Spider-Man: Homecoming got the right actress, as Zendaya was perfect as Michelle. We're looking forward to seeing her again in the forthcoming sequel, assuming that she's not joining the endless list of cameos in Avengers: Infinity War.