One axis of the antenna will almost always have wires 2-3 feet apart. Anything larger will have a hole in the middle where the tag will not be detected.
There are some exceptionally high performance designs that customers have discovered including 9' square and 6' x 15' loops that read 23mm and 32mm tags at a good distance. These antennas are made from multiple turns of thinner wire with the wire spaced evenly like guitar strings. The distance between wires is critical for these to work and can require experimentation to find these spots.
Antennas for the 12mm tag require extra care to get good read range and are often smaller than tags for larger tags that have a more powerful signal.