It has to match the furnace
Central air shares the furnace's blower and ductwork. The indoor coil sits on top of the furnace, and the blower has to push enough air across it. A new outdoor unit hooked to an old coil it wasn't designed for won't reach its rated efficiency, and it may not cool as well as the one it replaced. That's why the coil is usually replaced along with the outdoor unit, and why the technician checks that the furnace blower can handle the new system.
Bigger isn't better
Ontario's Building Code designs cooling across Ottawa for a humid 30 °C day. An oversized air conditioner cools the house quickly and shuts off before it has pulled the moisture out of the air, so the house ends up cool and clammy. A system sized for the house runs longer, steadier cycles and keeps it drier. Sizing comes from the house itself: floor area, insulation, windows and which way they face.
Where the outdoor unit goes
On a suburban lot the outdoor unit usually goes beside the house, and placement matters more than people think. It needs room around it for air and for service, should sit away from bedroom windows and the neighbour's, and shouldn't be under a roof edge where snow and ice slide off all winter.
Or a heat pump in its place
A heat pump goes in the same way as central air and cools just as well, but it also heats the house through most of the year. If you're replacing the air conditioner anyway, it's worth comparing the two. See heat pumps.