Short answer There is no scriptural rule that “men must use a round charula.” The Jain texts prescribe using a soft rajoharaṇa (charula) to gently clear the sitting place before pratikraman so that tiny lives are not harmed. The exact shape is not fixed in scripture.

A bit more detail

• Purpose: The charula/rajoharaṇa is a soft wool (or similar) brush used to clear the spot and your path with utmost ahimsa before, during, and after pratikraman.
• What scriptures say: Avashyaka traditions instruct śrāvaks/śrāvikās to use rajoharaṇa and muhapatti for carefulness (samiti) and restraint (gupti). They do not specify a “round” or “square” design.
• Why many men use a round one: In many Śvetāmbara communities this became a practical convention, not a rule.

1) Handling: A small round charula is easy to hold and rotate lightly in a circular motion, which helps in gentle, minimal‑pressure sweeping. 2) Uniformity in sabhā: It creates a simple, uniform kit for males (round charula, muhapatti, and pustak). 3) Convenience: Compact size fits easily with the pratikraman book and cloth.

• Women’s charula: In some places women use a slightly flatter or rectangular rajoharaṇa; in other places both use the same shape. Again, this is local custom.

Tradition notes

• Śvetāmbara: Laypeople commonly use rajoharaṇa and muhapatti. Shape varies by sangh/region.
• Digambara: Monks use the piñchhī (peacock‑feather whisk). Lay pratikramaṇa generally emphasizes inner repentance and carefulness; a specific charula shape for laypeople is not emphasized.

Bottom line Using a soft, clean rajoharaṇa with the right bhāva is what matters. Round vs. square is a community convention, not a compulsory rule.