I am planning to build a device that has a capability of early detection of a lightning. Also we want to measure the magnitude of the lightning and can anyone suggest a way to measure the magnitude of a lightning using magnetic flux induced by that lightning?
A simple RF inductor will work as an antenna. Or really any antenna for that matter.
Check out the AS3935 they use a simple RF coil to detect lightning. You could use your own radio detection circuit instead of the AS3935. Here is some info on the schematic with a BOM it has info on the RF inductor.
Here is some info on another group that builds antennas and stations for lightning
You can't reliably detect lightning before it happens. You can detect the conditions that make lightning likely.
A common method for assessing the immediate danger from lighting is a field mill. This is a device for measuring the local strength of the electric field. You might think that measuring the voltage of two plates some distance apart is all that's needed, but it's not so simple. The problem is that any method for measuring the voltage will eventually bleed off the charge.
A field mill is two plates, but they rotate so as to regularly swap which one is high and which one is low. By doing that, you get a AC signal. That signal is still high impedance, but by constantly flipping the plates, you get a continual signal, even if the charge is drained in the process of measuring.
The most practical method of detecting lightning is using a VLF radio receiver with a directional antenna.
Most of the RF energy released is under 50 kHz and there are multiple products that provide both amplitude and direction sensing.
There is also considerable research in this field (though why you appear not to have looked this up is troublesome). Perhaps start here.