I don't think the 30cm width whether circular or rectanguilar is particularly good in terms of either the player or group being able to visualize it. It would make more sense to make the width of the rectangle the diameter of the disc being used as a marker so everyone could see it (but leave the rectangle 30cm long as it is today.) Eliminate marking with a mini (unless player only wanted a 7-15cm wide stance zone.) Players would be allowed to use a different disc as a marker if they wanted to throw the one on the ground. They could use any disc in their bag as a marker so they may want to carry a Zephyr (24cm) or UltraStar (27cm) as a marker to get a little more width. The max diameter allowed for PDGA Approved discs is conveniently 30cm so no one would be able to exceed your 30cm wide proposal.
Rater of the tossed arc.