Hello Himit,
Let me try to answer your questions.
1.) Yes, a bigger antenna typically gives higher read range, if also the tag antenna has a decent size. In other words: increasing the reader antenna should give a better coupling in "far" distance - e.g. 10cm. If you use small tag antennas, the coupling in far distance might be less.
One possibility could be to try out a wire wound antenna with our discovery, X-nucleo or eval boards.
2.) Yes, you are absolutely right. It is recommended to stay within 0.5uH to 1.5uH - which does not mean that outside this range it will not work. Another important parameter is the DC resistance of the antenna which will also have an impact on the Q-factor. The DC resistance can be lowered (antenna Q-Factor increased) by greater pcb track width. The system Q can afterwards be lowered by adjusting the damping resistors.
Additional information can be found in AN5276.
3.) Not sure if I understood your questions correctly, but the EMC inductors can have an impact on the overall performance. Here the recommendation is bigger inductors have typically higher Q-factor and lower DC-resistance.
In general you will have to measure the antenna parameters and then adjust the matching components and capa voltage divider according to AN5276.
4.) You should go for the maximum (easily) possible supply voltage (e.g. 5V). Also the reader antenna should fit to the tag antenna size. Please consider which NFC technologies (ISO14443-A/B/V or Felica) and bit rates you require. Only maintaining 106kpbs (ISO14443-A & B) can have a bit lower communication speed but can boost your communication distance (higher Q-factor => AN5276).
Our ST25R3916B-EVAL (https://www.st.com/en/evaluation-tools/steval-25r3916b.html) board features a 66x66mm antenna. It should easily achieve your 4-6cm requirement (based on the used tag antenna size) using ID1 size tags.
It could be a good starting point for your design.
Please let us know if you have additional questions.
br Travis
