table size

I'm trying to decide whether it is best to use 1 big table or some smaller
tables with less rows for a database. Although Postgres doesn't have a hard
limit on the number of rows, I plan to be inserting, into the biggest and
the next largest tables, around ten million and two million rows,
respectively, a day. With good index choices, when can I expect the number
of rows to affect performance in a large way? Or, will it be best just to
leave all the rows in one continously growing massive table?
-Adam