I have designed and built metal detector electronics for my own use but never made a large coil detector.
The first issue is whether to use a VLF continuous wave, or Pulse Induction (PI) design.
Most systems like this use a PI design because the coil is just a simple loop.(mono coil). VLF systems that drive the coil continuously, need a design where the net magnetic flux into the receive part of the coil is carefully nulled to zero within about one part per million by careful mechanical design. This usually takes the form of a double D where the loop is a figure 8 with overlapping section in the center, a concentric coil where a big transmit loop has a small reversed coil in the middle, or a design where the transmit coil is horizontal, and the receive coil is vertical. In all cases, the strong transmit flux is approximately zero at the sensitive receive coil. In addition, there is an LC circuit added to further null out any residual flux due to phase changes.
Any mechanical flexing of a large coil mechanically nulled like this would disturb the nulling and give a false signal.
So unless you want to just line up a bunch of VLF detector coils, probably the PI approach is better.
No coil nulling is needed for PI since the large transmit pulse flux is finished when the sensitive receive section is on and the system listens for signals from the target's induced eddy currents.
If you already have a PI system, you might just be able to swap in a larger loop of similar inductance. ( Larger area, fewer loops).