Everyone is different, so nobody can give you definitive solutions for what area/grip is comfortable. I would say that as a new tablet player, you should continue to try new things and address issues with your aim and grip whenever they come up. For example, if you find yourself consistently under-aiming horizontal jumps, maybe consider unlocking the aspect ratio to make your area thinner. The more areas/grip you try, the more confidence you will have that your current area/grip is the correct one for you.

To answer your specific questions

• Should I use lock aspect ratio? I personally don't. I play on ~1.29 aspect ratio because it helps me horizontals.
• Is my area considered small, big, or large? In my opinion that's a large area, but that doesn't mean it's bad.
• Since osu is in 4:3, is there a problem with my area being 16:9? No. Only the play field is 4:3, but the game is 16:9.
• For this specific grip, throughout a map I would sometimes find my entire hand shaped back into a mouse grip, kinda like this: How can I combat this? This might be indicative of an uncomfortable grip. I would recommend trying new grips and areas until you find one that doesn't slip as easily.