Does the length of an iron bar changes when it is magnetised?
Theoretically yes, because iron is a magnetostrictive ferromagnetic material. The experimental measurement of this variation depends on your setup (iron bar size&shape, magnetic field strength, etc.).
That' okay but how CAN the length increase.
They are just aligning in the direction of magnetic field.
Who are "they" ?
Look here : https://www.physicsforums.com/showthread.php?t=76190
Let's see now. To magnetize iron you have to pass it through the field lines. Doing so generates an electrical current. Current meets resistance, generates heat, hysteresis. When you heat something what happens?
Mind you, with every day magnets, the effect could be so small that it would be undetectable without some veery precise instruments.
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