When I started in the Sims 4, I just built on the smallest lots 15x20 - like Nookstone. It's less intimidating. The lower floor could be kitchen / dining, lounge, bathroom. More than that would get cluttered. Have at least 2 bedrooms in the floor above and a bathroom. And that's a house.
That said, perhaps my altogether most perfect, amazing house fills up a 30x20 lot. And then I built the same house again on a 30x40 lot, and now there's a bit of garden space. A mistake I've made a few times is starting on a big lot and making the house way too big.