Tools
Diagonal inches lie. A 120-inch screen sounds "a bit bigger" than an 85-inch TV — until you see it's twice the picture. Compare true area and viewing angles, drawn to scale.
Picture area grows with the square of the diagonal. Going from a 65-inch TV to a 120-inch screen isn't "almost double" — it's more than three times the picture. That's why projectors feel so different from even the biggest TVs: your field of view fills up.
Size is step one. Next, confirm a projector can actually fill that screen from your room with the throw distance calculator, and that it's bright enough with the brightness calculator.