Assuming you have cPanel you can view what you're using in WHM under MySQL/MarieaDB, that's also where you can change which one you're using. Not sure in DA, but with SSH you can just type
The my.cnf file is in the /etc/ directory on cPanel servers. Use caution, lots of damage can be caused there. Size isn't as much of an issue as the hardware is regarding speed. I use to spend a lot of time optimizing databases, deleting old records, and more work thinking it would improve performance, but it really doesn't help much. These systems are designed to handle millions and millions, even billions to trillions of records. Tweaking the config file can help if you have unique needs, but for the average user that's looking to increase performance it's going to be hardware upgrades and optimizing queries and/or caching to reduce processing time.