Recent patches have completely changed how the economy works,, including the starting population and how population generates production.

Class refers to how many usable tiles a planet has.

So, it seems no-one knows. I read something from the devs saying that the goal was to create a game where the player had to use his/her imagination to create a story and immersion. This is why I wanted to try it and well now I'm not going to. Let me explain:

you start with 5/5 pop, you explore and find two colonizable worlds, you build two colonizers and take the recommended amount of people and now you got two worlds and one abandonned one (earth is now empty). Yes I know you can choose to take only 1 instead of the recommended 2.5 on a colony ship,, but that doesn't solve it, if you play on anything larger then medium size galaxy with normal settings you'll want to have more then just five planets with 1 pop (sure they'l grow eventualy if your approval is high enough but that can take awhile).

I could live with that if everyone started with that same limitation (still wouldn't be able to make it immersive though) but that's not the case, either I choose prolific or some other race will have it and they or me will wipe everyone else from the galaxy! Prolific gameplay: take 2 pop and colonize a world to get 3 pop there, build colony ship and go get 2 pop from the new planet, colonize a third and get 3 pop, rince and repeat to conquer the galaxy all you need is adminstator points, and that can be solved by taking the trait which returns administrator for colony ships (forgot the name)! Oh and don't forget to take first level of benevolant for your free extra home world (unless that has changed, I didn't test it) with a 5 pop colony ship.

Now I know this was possible when you started with 10/12 pop but at least then everyone could colonize 8-10 worlds by mid game which is enough.

I'll probably be wrong about this (not an experienced gal civ player) but it just doesn't sound like fun. Prove me wrong please!

Same here.

When playing synthetics,  colony ships with 0.5 pop, is the way to go during early colonization. 

Thank you for the mathematical solution, now we got that and a game-exploit (prolific and or benevolant and colonizer). All we need now is a fun and or immersive solution

Currently in Crusade (playing Terran and a couple of other races that are basically similar), they have growth rates that are sufficiently high as to take more population in a colony ship in the beginning.

A couple of considerations:

1. You start with a 5 pop planet at about 60% approval. Ideally, the best is to keep a world at 75% or better - the bonuses make a different.
2. Raw Production (the basis for everything else - Ship, Social, Wealth, and Research) is more or less driven on a 1:1 basis by pop.  That is, 1 pop = 1 Raw Production
3. You don't want to drop your homeworld's Raw production much, because it's effectively going to be the only planet doing much other than building it's own improvements for at least 25-40 turns. 
4. That said, due to the approval issue, somewhere around a 4 pop homeworld is best for the start. Which means, everything above 4 is available to be loaded on colony ships and populate new worlds.

Generally speaking, here's how I play for the first 25 or so turns:

1. Build a Shipyard right away. Some people rush buy it, I prefer not to (saving the cash for something else). But either way, the very first thing you build should be a Shipyard.
2. If you're a species that has a scout or survey ship, go start looking at other star systems RIGHT AWAY, from turn one. Your primary mission is to find a habitable planet for your first colony - everything else is secondary. Don't go out of your way to grab anomalies - they'll be there for later.
3. After you build the Shipyard, build a few more improvements on the homeworld; which ones are up to you and the particulars of the planet. I tend to go for the Space Elevator as #2, and then it varies according to the bonus tiles and arrangement of stuff.
4. Start building a colony ship as soon as the shipyard pops out.
5. When the colony ship pops out about turn 10-12, load it with (1) population. Send it to a habitable planet nearby - I tend to go for ones in other systems (e.g. NOT Mars, unless I haven't found anything else). 
6. Build another colony ship; rinse, repeat.
7. I tend to throw in a custom-designed sensor boat (cargo hull, 1 engine, 2 range, and as many sensors as possible) after the 2nd or 3rd colony ship is built, so I can get to scouting the local star systems faster.

By this method, I generally can make a colony ship at turns 12, 18, 22 and then about every 3 turns afterwards. And I can keep them filled with 1 pop trivially, even on a race that's just ordinary in growth.  That's a little slower than others may do, but it gives me sufficient time to survey well so my ships go directly to known habitable planets, rather than wander around looking for a world to colonize.

Bottom line:  the math seems to be in favor of putting 1 pop on a colony ship. That's sufficient to get the new colony running along fine, and not depopulating your primary builder planet right away.

After your homeworld gets powerful enough to start making colony ships in 4 or less turns (not giving it enough time to regenerate population), I tend to colonize Mars, and thereafter aways launch with 0.5 population. I stop the ship off at one of the other nearby colonized planets, and pick up another 0.5 pop - which works out nicely, because they all have pop caps of 3, and they easily get to that.

In general, I find the Colony Ship Benevolent bonus useless - the Research bonus (150) is FAR more valuable early on, as it gives you at least 2 techs virtually immediately - I tend to rush up to Ion Drive as fast as I can, so I can grab stuff further away from me than my opponents.  And you often don't know where to send that colony ship anyway, as you haven't explored a whole lot.  Others may disagree, however, and it certainly does differ based on what kind of map you're on (it's much less useful on maps without a lot of habitable planets within 10-15 hexes).