If I understood what you meant correctly, the protagonist in this story is young, but still an adult. There won’t really be a “growing up” phase — he’ll look exactly the way he does in the screenshots from the start.
Some female characters, like his first companion, naturally look much bigger because of their body type/species.
That said, the protagonist’s private parts usually won’t be huge like in many AVN games. The focus is still not on “big size,” but on knowing how to use it well, and enduranc.
Visually, everything is still very well detailed, so there’s definitely no need to worry about that.