You can't resist a little experimentation here by yourself. You decide to test the room's capabilities first with something small. You focus on your toenails, willing them to change. As you stare, you smile as they turn a pastel pink.
Alright, you thought, why not go big next?
Focusing on your complete self, you close your eyes and picture what you want. You start to age backward several years, returning to nineteen, a bit more fresh-faced than before. You open your eyes and smile again as notice your pink toenails again. Your large feet crack and pop, the bones shifting and their appearance changing. They shrink down to a woman's size 7 and become much more slender and dainty in appearance. Next your legs start to become smooth, the hair on them retracting as they take on an inviting, feminine contour.
Then you gulp as you focus on your genitals. You feel a churning in your gut and loins, as your parts are sucked back into you. Your knees quiver as your plumbing gets reworked and the lips of your new vagina open up.
Resisting the urge to probe around there for the moment, you imagine having hips and ass of a cute girl. There's a slight groaning and shifting as your hips press outwards, and your posterior clenches and releases in three short bursts, adding muscle and shapely fat with each pulse.
Next, you focus on having a belly that is flat, as your waist simultaneously sucks itself in, helping to form some of that hourglass shape.
Reaching your chest, you take a deep breath and decide on giving yourself a fair pair of assets, c-cup size. It's the same size as your girlfriend's breasts. You notice that your chest begins to feel heavier and heavier. When they are done changing, you jiggle your body and feel them bounce. You smile down at them, watching as your nipples and areolas grow a little larger, a pleasant tingling sensation erupting from them as water from the shower hits them.
Your arms slim down much like your legs did, and your hands become smaller and feminine, the same pastel pink polish that graces your toes washes over your fingernails.
Then you start to feel your neck getting thinner and slightly longer. You will all your facial hair to disappear as your face becomes far more feminine, your jaw cracking slightly as it reshapes. The blond hair on your scalp straightens itself as it lengthens, growing down to just above your shoulders. Next, you imagine your voice going up a few octaves, and becoming softer and girly.
In a quiet voice, you say "Testing. Oh, I like this!" You think your new voice sound cute.
Finally, you will your frame to change. You go from around 6'1" to 5'5" in about 15 seconds. You stick out a hand against the wall of the shower, steadying yourself as a small wave of vertigo passes before your mind adjusts to your new height.
In the shower, you finally decide to pee. It feels a lot different than when you were a guy. For one it runs down the inside of your legs, and down your ankles. You feel like that's a lot less convenient than when you were a guy. Thankfully the running water makes quick work of cleaning that off.
Your changes complete, you want to try out your body. You stand there in the warm shower and begin to masturbate. Instantly, you find that it feels so good, far different from jacking off as a guy.
Despite your inexperience with your new form, in your excitement you manage to cum in just five minutes, leaning against the tiled shower wall, head thrown back. Your eyes were closed, a look of uncontrollable pleasure on your face as wave after wave of pleasure ran through you, your body spasming slightly with each one.
As the glow fades, you breathe heavily as the warm water continues to beat down on you. You think about going for round two but decide to stop, realizing you've spent a lot longer in the bathroom than you initially intended. Your girlfriend would be waiting.
You wash your body, shuddering involuntarily as you brush soap across your tender nipples. After that, you wash your hair and condition it. Then, you rinse, and turn off the water. You step out of the shower into the steamy bathroom, grabbing a towel as you do and you begin to dry off.
Once dry, you look for your old underwear. You find the blue boxer briefs and focus on them being pink panties, same color as your nail polish. After they shift fully you pull them on, enjoying how they slide up your smooth legs, then look at them after feeling how different they are; tighter and more supporting in a way that feels nice now that you don't have junk being squashed down there.
You toss the rest of your clothes in the hamper and pull a purple satin robe off a nearby hook. After shrinking it to fit your new frame, you steel yourself and head back towards the main room, imagining the look of surprise your girlfriend will have when a cute blonde college girl walks out of the bathroom.