If you're choosing a drop-in sink, this information should be provided in the specifications for the product. The cutout must be large enough to accommodate the sink bowl(s) but also not bigger than the lip of the sink, which overhangs the cutout and holds it in place.

If you're choosing an undermount sink, the sink will install underneath the counter, so your cutout size should be about the same as the total, combined dimensions of your sink bowl(s) for a clean look. Your countertop must also have a finished edge around the cutout, since top of the bowl installs below the cut, leaving it exposed.