If you find the number of ohms per foot (it will be a very long number) then you can calculate the length of wire you require.
Here is a web site that has some figures. Google will reveal many more if this doesn't cover the type of wire you're using.
First determine the voltage and maximum current from your power supply. (Let's say it's 12V at 5A). For safety, we'll try to limit the current to 4A (about 80% of the rated current).
You then use ohms law to determine the total resistance required. R = V/I. In this case R = 12/4 = 3 ohms.
Now we go back to the wire.
We check that the wire can carry the current. Look
here. Use the value in the second-last column, and it MUST be greater than the current you wish to draw (or you need to recalculate for a lower current). Also you want to have some leeway in these figures. Your turns of wire can't get rid of heat very well, so pick something that can handle twice the current you require. In this example, I would want to use 22 gauge wire or thicker.
Let's assume I was planning to use 20 gauge wire. The table tells me that it has a resistance of 10.15 ohms per 1000 ft. The other web site I gave you agrees, and gives me the resistance per foot (which may be easier to use). It is 0.01015 ohms per foot.
So now I know I need a total resistance of 3 ohms and the wire has a resistance of 0.01015 ohms per foot. I need at least 3/0.01015 feet. That is 295 feet.
Easy!
If that sounds too much, then you might try a finer wire. Let's say you pick 28 gauge. It can safely carry 1.4A, so lets go for 1A.
From the calculations above, you need 12 ohms. The resistance is 0.0649 ohms per foot, so you need at least 184 feet. This doesn't sound a lot less, but the wire is much finer and there is much more on a reel. Also when wrapped around your steel rod, it will be much more compact.
Also note that it is important that the wire be insulated. Often fine wire like this has enamel insulation. Bare copper wire will short out and it won't matter how much length you use.