Original question:
Lucifer,
In general, the larger the size of the coil, the farther the distance you will be able to measure. A rough rule of thumb is that the maximum distance can be about 40% of your coil diameter. This makes more sense when you look at the following graph from the LDC Sensor Design app note. A larger shift in L is easier for the LDC2114 to detect and will make your design more robust.
http://www.ti.com/lit/an/snoa930a/snoa930a.pdf
You might also find this Target Coil Design app note helpful:
http://www.ti.com/lit/an/snoa957a/snoa957a.pdf
In summary, look at the mechanical constraints for your coil and target, then choose the maximum coil diameter size that will work. You can then use that value for the Webench tool.
