With what he'd seen, one thing that seemed fun is being the perfect size for a type of sleeve on a cock. He looked into the average sizes on dicks in comparison to some of his own holes, figuring out approximately what the smallest he could set it to would be.
Looking through a few different sources and averaging the sizes, he went just slightly below the height needed to fit perfectly on the average, since he'd be the perfect size and capable of also semi-fitting inside a girl of around the same age. For good measure, he also reduced his age a bit; Not too far, but he went down four years, getting him just out of teenager range.
After overlooking the settings once again, he decides to set the time limit to last a month. Not having to worry about the time limit *As* much would probably help his sanity, and allow him to have some more, slower fun.
He was about to initiate it, but then decided to add a bit more specifications. He made it simple, adding in that he'd be able to hold something of any size within him, and his body would stretch a bit to support it. Not that it'd be painless, or comfortable, but he wouldn't be in any direct danger if something ten times what he's intended to support were to be shoved into him
Initiating the transformation, it doesn't take long for Jeff to begin shrinking down. His clothes don't shrink down with him, so he ends up fighting with his shirt as he grabs onto his desk, not wanting to fall all the way down just yet. His pants and underwear fall off, and he manages to make his way out of his shirt as the transformation concludes...