This is a Loader mod. When activated, it will not apply any immediate change to the SDT scene (the girl's penis will not spontaneously grow or shrink). You must fiddle with the slider in order to obtain a smaller or larger penis.
The modified penis size will be included in your charcode (and can be incorporated into a Character Folder), but the SDT game will reimpose its normal min/max constraints when loading characters. If you wish to play with unusually-sized penises routinely, then you should include hpsri.mod in your Mods\$INIT$\Mods.txt declaration.
Some animtools positions will apply a custom maximum penis size. However, this restriction applies only to the male character. The female character's penis is purely decorative; it is not really involved in gameplay and therefore it probably won't be suddenly resized in order to fit into a scene. Of course, you may find it necessary to manually shrink an oversized penis because it's creating graphical oddities (e.g. poking into the male character's abdomen).
The coding of this mod of very simple. It will work with both penis variants (Penis A, Penis B) as well as the Strap-On; it will presumably work with any custom girl-penis mods as well. Because this mod employs uniform x/y scaling on static sprites, the outline of the penis will become abnormally thick as the penis approaches maximum size. There are no plans to correct this flaw.