I tried using medium sized tips for both my ears but the left AirPod wasn't stable and kept coming out. So, I tried a small sized tip for my left ear and medium sized tip for my right ear. Now, they are stable in both ears. The ear tip fit test shows a good seal for both ears in this configuration. However, I'm concerned whether the difference in sizes can affect my ears because I listen to them for long periods daily and will probably do so for a few years at least.
Symmetrically sized and placed appendages, organs, etc, of the human body does have measurable differences in: the comparative size of nostril openings; breast size and positioning (without surgical intervention, one breast is usually a bit lower down on the body;) leg and arm length; and ... ear canal diameters. You have proved this empirically! Congratulations!
I do caution you, though, not to rely on earbuds solely for listening, especially for long, continuous periods of time, as you have indicated. They can push ear wax back into the ear canal when used too often (it happened to me.) Incidental high-volume audio glitches can also cause damage to your hearing.
I prefer small Bluetooth/wired ear-cup headphones to ear-buds now. I only use my Bose Noise Cancelling buds when flying.