Here is the building that currently sits at 3004 Vine Grove Avenue in St. Louis, Missouri. And when I researched this house on real estate sites, it turns out it was built in 1892. So this is definitely the house Miss Lillian is living in at the beginning of our journey through her postcards! It consists of 1350 square feet (not too big, not too small). And since it was built in 1892, it is only 14 years old, so relatively new for the Maguire family.
The house is situated in the Greater Ville neighborhood, and is not too far from many St. Louis attractions. To guess what this house might have looked like on the inside, I searched on the Internet for Victorian and Edwardian home floor plans, and found this, which has similarities:
You can find this on the PBS site for Life in a 1900 House (which was a very cool series, by the way!).