i currently favor a 4-way split of buildings:
1 part factory, 1 part research, 1 part markets, 1 part everything else (farm, morale, influence). plus a starport.
so a 9 tile planet gets a starport, 2 each of factory, lab, market, one farm, one morale.
and a 13 tile planet gets a starport, three each of the basics, a farm, a morale, and an influence.
for 5 tile, i've been doing starport, 1 each of the basics, plus a far, but i'm thinking about changing that to be more specialized, but with empire-wide ratios staying the same.
and i have, in general, been deviating from this scheme to specialize a little lately, but i do try to keep the 4-way split when tiles are counted up for the whole empire.
and speaking of "empire", change your government type. the Imperial setting works well enough for the early game, but it won't get you all the way.