Flatlander, how big is a typical laylight, and do you know if there any practical limit beyond which the weight of the glass itself requires that it be divided into several panels?

Well, there is never any such thing as typical in historic buildings, but I expect the size would only be limited by the strength of the framing. If you look at large stained glass windows you'll see some pretty thick support members, generally running horizontally across the window. [Word-related]The framing around each pane of stained glass (historically made of lead, now probably copper or something else) is called the came. For a stupendously big laylight I guess you could suspend it from above at key places to provide extra support, but you would need to divide it into several frames. I should add that I've never seen a "stained glass" laylight (as in individual single-colored panes of glass) -- they are more commonly painted, etched or frosted.