HI !
I'm interesting cheap camping with a big and extra beach ??? thanks!

There is a great camping in Sithonia, it's called Armenistis.Armenistis is situated between Vourvourou and Sarti, take a look on the map of Halkidiki. In this site you can find all the information you may need (www.halkidiki.com).

kontakt, armenistis@campsite.gr

Rent a bike for a couple of days, and make a round of the east part of Sithonia. you will find at least 10 -12 nice camping places there.
Armeninstis is great, with a very nice with a wide beach, but at the same time is very crowdy. Specially in the weekends it becomes extremely crowdy!
If you like diving underwater, try camping Zografou. A famous beach bar is also close there.
If you have a family with small children, camping Kalamitsi is also nice.
Avoid camping Lacara (last time i visited it was almost out of trees)..
In general, everybody will allow you to take a look from inside the camping before you decide to stay there.
Greek campings are safe, and almost in all of them you can find super markets to by what you need.
The fact that Armenistis camping is close to a head village called Sarti, is also an advantage.
Unfortunately, campings are not marked on the map, (at least not in all the maps) so thats why i suggest to take a ride and see them one by one.
Some more info:
A couple with a camp might cost about 20 Euro (without any vehicle)
Free camping is forbitten in Greece

Hope you will find these info helpful.