Be honest with yourself and also ruthless: will all the extra bedrooms or multiple reception rooms really get used? There exists a community of self-builders who are focused on building themselves a tiny house, defined as 16 to 40 sqm in size, for financial and self-sufficiency reasons.
And don’t feel the need to be bound by convention or tradition. For example, houses in the past had chimneys as that was once the only option to heat the home. However, it is now very easy to build a house that requires almost no heating system at all – so the actual need to have a chimney no longer exists.
Finally, what you have built will outlast you so bear in mind that at some stage in the future it will need to be sold. Some self-builders have built houses that are literally too big (think running costs, upkeep, property tax) to sell on.