Projection is an art, with the size and type of projector being dictated by the audience and room sizes, and partly by the availability of adequate hanging points. Each projector has a focal distance, which determines how far from the screen it needs to be in order to fill that screen, and this changes with screen size.
With larger setups using multiple projectors, the images must be colour balanced, not only to each other, but to the presentation material. In some instances, multiple projectors may be overlapped or 'blended' to create one large image. This is done with a combination of precise hanging and focusing, and special software which allows media to be run across multiple projectors.
At the 'back end' of projection, is all the equipment that creates the projected image. The average Mirage setup requires some form of seamless switcher (allows switching between two image sources, such as a DVD and presentation laptop, or to move back to the main presentation and a holding slide), multiple sources, such as laptop, DVD player and/or a live camera feed to screen, cueing monitors to allow the vision technician to view all sources that don't come with their own screens (ie: DVD players), a vision mixing console (if more than two sources are required to be mixed), a blackburst generator or genlock (if using multiple image sources with live camera), signal amplifiers, signal distributors and of course, hundreds of metres of specialist cabling.
And of course, the person running all this equipment, with years of experience, skill and timing, takes responsibility for equipment installed into your venue for the evening!