150mm is standard column size in concrete for most single story houses.
the maximum span i have usesd them for is about 3.5m but this was due to design features not due to restrains due to load.in the picture they are not only architectural but also support the 1.8m overhang for the verandah area.
for a small house like the one you wish to build then yes 150 x 150 would be sufficient. personally i would use concrete for the structural work and then use the timber columns around areas which people wil see.