Here's one that I found on Amazon. No heater. https://www.amazon.com/Homewerks-Worldwide-7141-80-Integrated-Ventilation/dp/B07X3Y6B1V/ref=mp_s_a_1_3?keywords=bathroom+light+fan+combo&qid=1687696033&sr=8-3&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc The longest dimension is 12.6". I searched for 'bathroom light fan 16" centers'.
There must be more -I know I got a whisper quiet one for our older home that fits between our 16" rafters.
The big problem sounds like the drywall. While doing drywall on an old ceiling isn't for the faint of heart, it's better to have to make a bigger hole than patch a hole that's too big.