The tension in a joist isn't proportional to the span, but to the square of it. So it becomes huge very quickly. As a practical rule one can take 1/10th of the span as the height of the joist and 1/20th as the width of it. Next applet calculates the weight of a marble joist according to a given span.

By increasing the span, joists become increadibly heavy very quickly. That's the reason that while using architraves, the span is always kept very moderate. It's not a problem to cross the width of a door, but for creating large halls one has to wait until the invention of armed concrete, around 1900.[br]The great hall in the temple of Karnak measures 5000 m². 134 columns are aligned on 16 rows. The middle ones are 24m high with a perimeter of 10 m. But as a visitor you still got the feeling just to walk in a forest of colums, rather than in a huge hal.