People with 40-50 tsubo houses, how do you find the size?

I found some land I'm interested in buying. It's about 100 tsubo and I'm looking to build a 40-50 tsubo house. We are a family of 3 with our second child expected in September. The architect we are working with told us we should do 40 tsubo to keep the price down because the land is very expensive and our house would cost around 60,000,000 yen if we do 50 tsubo.

Looking for opinions on how you feel about the size of your house if it is around this size. Is 40 a little too small? Just right? 50 too big?

Thanks in advance.