I know that the resistance of an electromagnet depends upon the nature, length, and diameter of the wire. Is it also depend upon the size and nature of the iron core on which the wire is wrapped?
The DC resistance is not affected by the core or the material the core is composed of. As you said: it is affected by the material, length, diameter of the wire. Note that temperature also affects the resistance.
However, the core material DOES affect the AC impedance of the winding.