How big should my sump be?

I just bought a 650 litre tank second hand with a built in overflow. The previous owner didn't have a sump and was using the overflow as a sort of built in filter.

So my question is how big/what volume should the sump be. I have a standard 3 foot tank if that would be ok and don't have much money to spend as it all went into the tank.

I could mechanically filter the water in the overflow just need the sump for biological filtering and for the heater.

This is my first big tank and some help on sump size and design would be great thanks

 

I think a standard 3 ft will be perfect size.

 

Congrats on the new tank.
I have a 660 L Buyo tank with sump. My sump is 110l which is similar to your 3 foot. You will need to look at adding glass panels inside and purchasing a return pump. The sump set-up costs me about R2000. Attached is a pic

 

A 3 foot tank is plenty for your 600l tank.
You will obviously need to perform some DIY on your 3ft tank, so I recommend you have a look at the thread about sump design in which Prof Belstedt gave some very good advice. And following his design, as a bonus you will save yourself some $ by not falling into the trap of going for a massive pump .....