Use the largest tank you can fit under the stand AND afford. Lol
There are dozens of sites explaining how a sump functions and diagrams for building them. There are dozens of threads on here with diagrams and step by step build processes.
But I'll throw the basics in here. Some else posted a price breakdown, and that should give you an idea.
Water will flow from the main tank down. If the tank is not drilled and sump ready, you will need either to drill it or install a hang on the back overflow box. Drilling is tedious and time consuming, but the results are cleaner and slightly more reliable. Overflow boxes (eheim makes a good one. It's what I use) work just fine but you'll need to make sure the flow rate is high enough to avoid air bubbles accumulating and breaking the siphon.
From there, the water enters the sump, which should be divided into four areas. Inlet, main chamber (I have a refugium in my main chamber), bubble trap, and return.
Using glass panes(baffles) and silicone, the tank is divided into these sections. The height of the baffles will determine the sump water level.
Inlet chamber does not need to be large. Simply an empty section for water to flow in. Main chamber is usually where all the equipment goes so its larger. I have my skimmer and heater in the inlet though, so my main chamber is about a foot wide and has a six inch sand bed. After that, you'll want three baffles close together. One set against the bottom of the tank (water spills over the top), one set an inch up (water flows underneath), and a third positioned like the first but usually shorter. These three trap bubbles and keep the pump from shooting them into the main tank. The remaining area after the bubble trap is the return section, where a pump pushes water back up into the main tank.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef