For anyone looking to change their dick/breast size in game it's just not possible however you can change them easily outside of the game without installing any third party software.
Simply go to Save Edit Online. This is a save editor (you can search for it manually if you prefer not to click links). First, export your save by locating the button on the bottom left of the pop-up screen where your saves are stored. Grab the exported save from your downloads folder (or wherever your downloads go) and drag it onto the upload box. You can also drag the file from the download bar (usually at the top right of your browser) or manually click the 'Upload File' button and search for it.
Next, a lot of text will appear—don't be intimidated. The code may seem overwhelming, but it's more bark than bite. Press 'CTRL+G' to open the search function, then search for "Psize" or "Bsize." If you have multiple saves, you'll get several hits, and you may need to determine which is the correct save based on context (like hair color).
Once you find the relevant code, you'll see the word 'small' next to Psize/Bsize. Simply delete 'small,' save the file, and then import the modified save by using the import button, located where you found the export button. (DO NOT DELETE THE "" THAT SURROUND SMALL)
Finally, check a mirror (or get into an encounter) to verify the changes were saved.
this is a temporary fix until more customization features are added in the future this currently works with v0.0.4 so this may stop working in the future
P.S. Technically since the size modifier for your cock/breasts isn't a number variable you can replace small with anything else like "large" or "massive" you could also technically put in "Knotted" or "Ribbed" if that's what floats your boat though it was meant for sizes so anything else may sound weird when directly referred to in game.
P.P.S the sizes are also beside the number variables of whether or not you have a cock or pussy if you set one of them to 0 it should remove the undesired part though this part is abit of a hit or miss on whether or not it works.