Just did the math. 15" 4:3 is actually bigger than 2x 10.5" iPads side by side. That's like 4x iPad minis. 🙄
15" at 3:2 will get you exactly 2x 10.5 iPads side by side, I believe.
If you want to stick to 4:3 aspect ratio, then 14" diagonal will get you a short side that's the same length as the 10.5's longer side.