I'm not sure about a max size of a "world" you could build in Bryce. I know after a while just moving the camera around is literally exercise....

On one project where I need to scale things somewhat accurately, and I didn't want to create a huge scene, so I based the start of the scene on a 6 foot tall male. I just resized an imported male character (mike 3.0) to be as close to 72 units tall as I could get him, thus each Bryce Unit would equal 1 inch, and then built everything around him based upon that.

So, when I need to, that is my own personal default, one Bryce Unit equals one inch.

Basically, I would suggest scaling everything down a little if it is putting undue stress on your computer.

*And, not sure of you know or not, but two other things you can do to help a large scene populated with lots of meshes, to not bring your cpu to its knees;

-Reduce the level of resolution on your wireframes.
Motion 8
Static 16
Selected 32

-Any mesh, in its Attributes, can be given the option of "Show As Box". This shows that mesh as only a bounding box, and your cpu/video card will not have to display its wireframe, thus quickly speeding up the navigation of large, poly-heavy scenes. (Good for when you have lots of buildings) Visually it takes a little getting used to, but it does help.

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