Notice that you're unable to shrink

Sophie: "Alright I need to figure out a way to make use of these abilities somehow... but first, gotta eat. Wonder if McGonards is still serving breakfast?"

You decide to shrink back to have a quick lunch, hopefully before the mayor rushes to the scene demanding an explanation on how and why somebody has built a new park here without their permission. However, concentrating onto your size again, you only seem to be able to grow taller.

Sophie: "Whoah-oh."

You concentrate more. You think about diminutiveness and puppies in tiny sweaters and mice and particle physics... no dice. You've grown to such sizes that your foot can just barely fit into the park that you just created. You gracefully step out of the entire city and into the ocean.

Sophie: "I'm sure I couldn't do damage to the city at any size, but I don't wanna mess around too much."

You look curiously towards your home city.

Sophie: "...or do I?"

This size offers some entirely new possibilities. There's an abandoned beach near the edge of the city that you've always disliked. It's ugly, the sand is coarse and uncomfortable and the waters are infested with blue algae. You decide to get rid of it. You wade over to it through the toe-deep shores, stick your hands through the bedrock beneath the beach and lift. The entire area rises into a beautiful, monumentous cliff. Anybody who saw it would agree that it's better like that.

Sophie: "Now, what else... Oh! There's an awful shoal near here that disturbs cargo and passenger ships."

You scoop up the large rocky shoal into your hands like gravel, and then spread it over your park, creating beautiful and artistic menhirs for people to admire.

Sophie: "Then, near the industrial zone when you enter the city, there's that crummy intersection that nobody knows how to drive through. My aunt had to be hospitalized after some jerk drove into her car's side at 55 miles an hour without the right of way!"

You walk over there skirting the edges of the city, careful not to step on anything. Parks are nice but having too many of the same shape and size could get dull. Once you reach the intersection, you stomp on it with your foot. Underneath, a neat roundabout is revealed, complete with a granite statue of yourself at the center.

Sophie: "Oh. My god. I just realized I can create train tracks that run north-south, as opposed to east-west! I could cut out hours of time visiting my other aunt in Bakersfield! If... If I'm ever able to shrink back down, that is."

Whether or not you'll ever make use of it yourself, you decide to litter the whole of US with passenger railways. By trial and error, you find out that you can create rails by simply drawing onto the ground with a finger. Thusly, you walk due north hunched down, dragging along two fingers on the ground at a speed of almost a mile per second. In a matter of hours you've created 6 thousand miles of high-speed railway, along with thousands of parking lots, parks, attractions and even diners, which sprung out from your footprints.

As you return to your home city, you realize that you've grown considerably during your quest for infrastructural improvement.

Sophie: "There's one more thing I could try. Hopefully my intentions are enough to guide these powers into making what I want."

Your city, like many American cities, has a housing crisis. Apartment buildings and whole suburbs remain vacant while people who can't afford them are crammed into outright slums or **** into homelessness, all because of greedy 'investors' who hoard real estate and hike their prices. You're going to punish them.

A good quarter of your city is composed of exorbitantly priced suburbs. You stomp out all of it, every last building. In their stead are... the exact same buildings, but now the deeds are owned by their inhabitants, and not by landlords. You're unable to check whether this change has actually occurred, but since no other change is apparent, you believe that it has.

Sophie: "I'm doing so much good! But if only I could shrink back down and see if people actually like what I've done..."

You need to try and shrink back down.