It's easier and cheaper to put in a bay the same size or smaller than the old window. However, a larger window will bring more light into the room and create a greater visual impact inside and out. Often, the bay sash match those on the other windows on the home, but they don't have to. Pick the style you like best. Taller, narrower bays usually look better with casement sash.
Shorter units are most attractive with double-hung sash.
You'll also find that the center window of a bay usually is fixed. But it, too, can be ordered with operable casement or double-hung sash in smaller sizes. Bay windows must be supported from below with braces or from above with steel cables bolted to the overhead framing. Both systems work well, but support cables are much less obtrusive because they're hidden inside the window frame. Not all manufacturers offer support cables, but they can be bought separately and adapted for use on virtually any bay window.