What's the capacity of a fuel tank on a 2001 Ford Ranger edge

19 gal.
http://www.cargurus.com/Cars/1999-Ford-Ranger-Specs-c193

The most common issue is that the fuel tank fuel level sending unit is defective. Before having the tank pulled to replace the level sensor have the problem checked to confrim my guess is correct.

Pretty sure its your sending unit, wich is on your fuel pump inside gas tank

I believe there were two options 15-22 but i'm sure you can find out by filling it when its almost empty

EVAP System component failure,such as a fuel
tank vent.
You should have a Mil Lamp light on dash
and codes to get close to the issue
Need professional help,
don't need a dealer,unless you have deep pockets

under so circumstance should it leak out. Either the filler neck is leaking or the seal for the fuel pump. You will need to lower the tank to check it out.

special tool (about $15 at any parts store). It pushes in around the fuel line to release a clip inside , then the fuel line just slides off. Sometimes takes some wiggling and jiggling.

There are two metal straps that hold the tank in the vehicle. First you will need to disconnect the filler neck from the tank and any other black vent hoses that will pose a problem. Place a floor jack under the fuel tank and remove 2 bolts (1 in each strap). Then carefully lower the tank. You will need a special tool for the actual fuel lines where they are connected to the pump. It is called a quick disconnect tool. Once the tank is free of the vehicle you will see the pump housing on top. Most likely has about 8-8mm bolts that will need removed-and there is your fuel pump.. Labor guide calls for 2hrs -normally takes about 45mins to complete..

You may have a bad idle control valve. If you want to replace fuel pump yourself you will have to remove bed of truck or fuel tank.