It seems that you should first choose the three-yuan, three-in-one or comprehensive plate according to the martial arts! The size is convenient for your own use, but it's too big to carry around. If it is too small, it can only be used as a decoration if you are not basic enough. . . . . So I feel that mid-sized disks from 6.2 to 8.2 are better, and at least 22 rings of content should be enough!
It seems that choosing a compass is quite particular.
It depends on the workmanship and material.
Is the inner disk rotating flexibly?
See if the handwriting is clear.
See whether the magnetic needle is flexible or not, whether its pointing is accurate or not, whether its rotation is sensitive or not, and how long it takes for the fixed needle to stabilize.
Does the seabed line in Tianchi mean due south and due north (the center of the meridian of the land)? The north side of the seabed line should point to the imaginary and dangerous coincidence of the twenty-eight constellations of sewing needles! The southern end should point to the Zhangsu third degree of the twenty-eight constellations of the stitch. This is directly related to the craftsmanship!
If you have an outer disk, check whether the ten paths of Tianxin are at right angles of 90°, and whether the outer disk is square.
Is the size suitable for you?
. . . . . I haven't thought of anything else yet, I'll add more when I think of it.