Fresnels must be at least 20 mm from the LCD, or you will see the fresnel rings in the projected image.
The size of the fresnels depends on what the light is doing as it passes through the LCD. If all the light is perpendicular to the LCD, (in a split fresnel design), then both fresnels should be about 1 cm larger than the LCD on all four sides.
If you have a non-split design, with both fresnels before the LCD, then they may have to be a bit larger. The light will be traveling through the LCD in a converging cone. The fresnels have to be just large enough so that the converging cone lights the entire LCD. If the fresnels are close to the LCD (ie. 20 mm), then the fresnels do not have to be much larger than the LCD. If the fresnels are very far from the LCD, then they will have to be much larger. Make a full-scale drawing to see.