The highest resolution of the Optoma S331 is SVGA (800x600). That's 77% less resolution than a 1080p (1920x1080) multimedia projector, which is the most frequently found resolution.
A SVGA (800x600) native projector resolution is in fact the worst resolution across all multimedia projectors that are a comparable age to the S331.
Across similar types of projector the best resolution is WUXGA (1920x1200).
It may also be worth considering one of the other multimedia projectors that have a resolution of SVGA (800x600) currently available including because of it's better review score, the Optoma S321 (£209.99).