I am a licensed journeyman electrician.
The device is rated at 20.9 amps, not 20. You go to the next size breaker, a 30 amp. It is 240 volt so it requires a 2 pole, 30 amp breaker. The wire size for 30 amps is #10 copper or #6 aluminum. You can legally, according to the NEC, use a larger size wire, but the overcurrent protection must match the rated size per the nameplate. Period, end of story. In addition, since the instructions call for specific wire size, etc, you might want to heed them to protect your warranty, and potentially your or your loved ones life. But its your decision. Listen to advice from a professional or take advice from amateurs, your call.