Buying a projector starts with a tape measure, not a spec sheet. Two numbers decide whether a projector works in your room: how big you want the picture, and how far back the projector can sit. Everything else (brightness, resolution, price) comes after. Answer three questions below and you get the throw distance you need, the kind of projector that fits, and how bright it should be.
Every projector has a throw ratio, printed in its specs as something like 1.2:1 or 0.69–0.83:1. It is simply the distance from lens to screen divided by the width of the picture. A ratio of 1.5 means the projector sits one and a half picture-widths back. A 100 inch 16:9 picture is about 2.2 m wide, so that projector wants roughly 3.3 m of room. A ratio under about 0.8 is called short throw: the same picture from under two metres, which suits a living room where the projector sits on the media unit just below the screen. Anything near 1.1 to 1.5 is a standard throw, which suits a coffee table or ceiling mount at the back of a typical lounge.
Many projectors have a zoom lens, which gives a range of ratios instead of one number. That range is your wiggle room: a 1.13–1.46 projector can fill the same screen from a fairly wide band of distances, which is very handy when the sofa and the shelf are not where you would ideally put them.
Brightness is measured in ANSI lumens and it gets spread over the whole picture, so doubling the screen width quarters the brightness per square metre. A modest LED projector around 500 ANSI lumens can look lovely at 80 to 100 inches in a properly dark room and washed out the moment a lamp is on. Rooms with curtains and some glow want something in the region of 1,500–2,500. If daylight stays on, you are in the 3,000-plus club, and even then a projector is a compromise next to a TV. Be wary of marketing numbers on very cheap models: look for ANSI lumens, not just "lumens".
The picks below are three BenQ projectors at different price points, chosen because their throw ratios and brightness differ in the ways the picker above describes, so one of them is likely to match your answers. Prices vary by shop and day, so the picks show current ones.