Herself, but with just a few changes.

A slowly rotating 3d representation of your girlfriend is visible next to the stats, much like the character creation/modification interface in a video game. Scanning through the options, you skip over Name, Age, and Gender.

You compare her current height to the idealized version of her in your head, and adjust it so she's 5'6" tall.

You've always wished her hair were longer, but you know she struggles to keep it under control, so you make a couple changes - her hair now has a silky nature, making it easier to tame, and reaches down her back when standing to brush the top of her ass.

You've always loved her eyes, and have no desire to change them.

You notice that her weight was automatically changed with the height increase, currently at 115 pounds, and wonder if changing her build will modify that as well. Experimenting a bit, you discover the build options are just generalizations, and you could individually tweak various aspects if you wished.
Not wanting to risk some of the...disturbing...results you've seen in video game systems like this, you go the safer route and change her build to "Curvy" noticing her weight increases once again to around 130 pounds.

Your pants also seem much tighter as you view the changes. Her breasts seem to have automatically increased in size to keep things proportional, and you note that stat now reads "D".

Unable to resist the urge, you tap that stat, which expands into a bewildering array of details, organized into subgroups such as size/shape, feel/weight, skin colors, sensitivity, and so forth.

Testing out things in the size/shape menu, you manage to increase her breast size to an unusually firm-looking E, enough that it would make people wonder if she had gotten breast implants, but without quite looking fake somehow. This makes her overall body appear top heavy, and you go back to the build stat hoping to find additional options as well.

You do, in an even larger and more complex array. However, they're also grouped, and you find the stats relating to her lower body, trying to adjust things so she looks more balanced. Either your growing arousal is warping your understanding of the word, or you just don't care anymore, and the resulting changes make your girlfriend's image appear almost unrealistic. Her ass is now huge, but her hips and thighs have expanded to match, hovering just on the edge of being beyond realistic.

Suddenly, you notice a flashing + sign near Relationship Status: Tapping the button, you see a vast list of emotional and relationship stats. It would take you hours to go through everything, but a quick scan reveals some concerning things. Some of the areas are flashing as well, and expanding those menus reveals that this new body will cause your girlfriend to attract attention from the most popular students at her highschool, giving her the option to date someone far more physically attractive than you are. It also reveals that despite your relationship not reaching that point yet, she won't enjoy sex with you as much, since she has a secret love of anal penetration, and your normal-sized dick just won't reach past her now enormous ass.

And the mutual groping you've done has led her to think you won't measure up.

Somehow this version of your girlfriend has become an anal size queen, although the stats indicate she will (if you get that far) still enjoy sex with you in more traditional ways.

You desperately want to have sex with the fantastical version of your girlfriend now visible on the screen, but the thought of being unable to satisfy her deepest desires (pun intended) makes you hesitate.

Do you change her more?
Do you stop here and hope for the best?
Do you change yourself to try and match better physically?