We are a family with 2 kids and 2 adults

How big should the sewage tank be?

Looking for a 600l is that big enough

What about overflow from it what to do there

Regards

You should say where your location for good help. Anyway... the normal for upcountry is to put in a 3 ring cesspit and you need to empty once a year - or more. Try to at least get a septic system - where there is septic tank and leach field. Don't worry about size. If you get a good septic system, you shouldn't need to worry about that for many years.

600L will likely be too small.

You're probably looking at a 1000L at least, we have two 1000L tanks (3 bathrooms) which are the prefabricated black spherical beasts.

The outflow should run to a leech field (soak-away), the locals tend to run into whatever drainage is nearby which can lead to smells, particularly if your tank is too small.

If done properly it should never need emptying.

As Steve notes, what is your location, how far out of town?

Thanks for answering. I live in north east. I have read other places that many use a black 1000l tank and the overflow from the tank is flowing too tank making of concrete rings with the grey water(bathrooms and so on)

I'm using gutter on the roof. Can I lets the rainwater go through the grey water tank and then out to the channel or do I have to make 3 different system.

Black water system, grey water system and rain water system

Regards

We run our rainwater drainage through the grey water system, helps to dilute the effluent further. Yes, it then goes to a local, fast moving, water course (like everyone else in the village).

No smells so something is working properly, the locals also eat the fish from the river.