How does the (i) thickness and (ii) length of a fuse wire depend on it's current rating?

Why is it not advisable to use a piece of copper wire as fuse wire in an electric circuit?

An electrical kettle is rated 3 kW, 250 V. Give reason whether this kettle can be used in a circuit which contains a fuse of current rating 13 A.

A fuse is always connected in the live wire of the circuit. Explain the reason.