The main factor in choosing the size is how big is your wrist. The 42mm watch may look quite large on a petit women's wrist. Even then it may be just a personal preference. The second factor to look at is how well you can see the screen. There is only a small difference in the screen size however the larger size is much easier on my older eyes.
I was consisting the 38mm model for my smallish to medium male wrists since I didn't want the watch to "stand out". I opted for the 42mm simply because the larger screen is easier for me to see.
Check out this article for more help. https://www.imore.com/which-apple-watch-size-should-you-get
The best is to see them in person at an Apple Store or a Best Buy and try them on. If this is not an option you can cut out a mockup of the 2 sizes out of thick cardboard, the dimensions are in the iMore article.