This is not really my area, but maybe I can have a try at helping and people more knowledgible can correct.
Appologies if I lead you the wrong direction, or just plain guess wrong.
Seems a lot of RFID these days uses passive tags. That is, the tags are powered from the reader while nearby by magnetic coupling.
Maybe you have a way to tell whether a tag is being energised, so you can tell whether your problem is energising, waking the tags or signal strength?
I guess the 125kHz source and FET in your diagram might be creating a field in the coil that enegrgises the RFID electronics by some cunning means.
I don't have a good mental image of how coils and magnetic fields work, but I sort of picture a feild strength as a number or density of magnetic flux lines passing through the area of the coil. The RFID coil intersects a proportion of these flux lines and reciprocity gets it a current to its electronics and viola power. If the old system was 350mm diameter, I guess this created a flux density that was enough to energise an RFID reciever from its coil.
Now, you want to make a coil covering 1.5m. I don't have a good feel for the effect of a small coil on a large coil, but if we assume the small RFID gets a proportion of the field lines related to the area, and area is pi * diameter, seems as a first approximation, you might need 1.5/0.35 = a little over four times the flux lines, or input drive, to get the same number of lines intersecting any one RFID in the area.
So, more power into the coil, by a factor of four, just to wake one up.
From memory coils producing fields are moviated by current and turns. If you inductance is way up from the diameter increase, you;ll have to change things to keep resonance if that's what you need and getting more current through it is also going to take some work.
Not sure about how the return path works. Could be the coil area affects the signal levels back too.
Hope this helps in some way.
Those of more experience; please feel free to correct.