The cap is +100%. The 50% stated earlier was as an example. There are 6 levels of miniaturization, giving 10%, 15%, 15%, 15%, 20%, and 25% respecively. Research all the way up the tree and you effectively double your ship size.
Brighton's observations on the population cap match mine, but I disagree with his conculsions on the usefullness of farms. Find a good planet with no farming bonuses and you're capped at 5 billion. A basic farm bumps that up to 9 billion. People = taxes, and you need tax revenue to turn production capacity into social and military projects (buildings and ships).
Stardock Forums
Won't the size's change as you research miniturization?
what exactly is the advantage to a larger population. Like Supreme Shogun said, more people = more taxes. You'll need extra people to mount an invaision at some point anyhow.
To be fair MrMorose, the difficulty settings are at the tail end of the manual, perhaps he just didn't notice them. I had to load up the PDF version to catch it. I'm afraid these old eyes just can't read the mini font they used in that table!