1. Cut Your Coat According To Your Cloth
Building a smaller house doesn’t necessarily mean it’ll be cheaper. There’s an economy of scale on every project, so as owner builder you must understand how to strike a balance somewhere in the middle to get maximum value per m2.
What does this mean in literary sense?
Few days ago, a client approached me that she’d like to build a house in Lekki and she’d prefer a bugalow because it is cheaper and doesn’t require a slab blah, blah…
Like this client, there are many other owner builders who based their decision to scale down their project on cost forgetting that for every house project there are site set up and running costs – preliminaries, administrative, logistics, etc.
The smaller the project the higher your running cost.

My recommendation to you is to find a sweet spot where the job is big enough to build efficiently, without it being too complicated and overly expensive that you go over budget.
Regardless the size of your project, some costs are inevitable and constant on the project.