You need an overflow of some type (drilled or hangon) which will bring your water from your tank to the sump, and maintain the siphon when the water flow stops.
Once the water is in the sump and finished with what you intended it to be doing (filtering), it is then pumped back to the tank and dispersed throughout.
The overflow should have a higher gph then the pump.
The PVC sizing will all be determined by the size of the overflow and the return pump.
As for what you put in your sump...that is the beauty of it...whatever you want. If you want your biowheels in there and you can make it happen...then put 'em in. Everything out of your tank (heater, etc) can go to the sump.
Sump size is whatever size if big enough to maintain a good level of water, and has enough room to accomidate your filtration items. And as mentioned above..bigger is better as it increases the total water volume of the tank.