I have read about people saying about furnace size, heat loss, etc.
Please leave all that.
Nope. That is the stuff you need to figure out before anything else. Start by defining your interior volume, and then specify your insulation. Assuming 450 C inside the box, and knowing the thermal resistance of your insulation, you can figure the power necessary.
Once you know the power, you can make an operating assumption about the wire temperature. Let's say the wire will operate at 900 C, which will allow 50% duty cycle. Using the date sheet for your nichrome alloy you can find the resistivity of the material, and determine the wire length and diameter.
At this point, you have a known wire length and resistance. Then, given the power you need (remember power?) you can calculate the supply voltage as Warren Hill suggested. Dividing power by voltage will give you the current required.
Then you will need to calculate the resistance of the wire at room temperature, and rerun your calculations. The previous numbers gave power and current at operating temperature, and the wire will dissipate a good deal more than that at turn-on when the wire is cold and has less resistance.
But first things first. The very first thing you MUST do when designing a system is to determine your requirements. In this case, the most important requirement is the power needed at operation - all else follows from this, and in order to determine it you must know the insulation values of your oven.
"Please leave all that" doesn't work.