Can I mix RAM sticks of different sizes?

Probably, but with caveats. Mixing a 16GB stick with two 8GB sticks usually enables “flex mode” — the first 16GB runs dual-channel, the extra 8GB runs single-channel, and real-world performance impact is under 5% for most gaming and everyday use. The bigger risk is compatibility: mismatched brands or speeds can cause instability, crashes, or forced downclocks. If the new stick matches speed and timings, you’re fine; worst case it doesn’t boot, but you can return it.