Circular building with a 20 or 30m diameter

Well, I need help:
I want to make a large circular building but as prim size is limited to 10x10x10m I'm having a problem...
I'd like to have perfectly round walls. so simulating a round wall bis lots of little box prims is out (an a terrible waste of prims as well). I thought of using cut on a cylinder to male a "cake" slice and then enlarge that back to 10m radius and to get my 20m diameter taht way.... but that doesnt seem to work either. the radius stays at 5m....
Does anyone have a sollution?

The only 'legal' oversized prim is a twisted torus (90 begin/end twist). But i doubt it can be used for the purpose you want it to be used for.

There are some 'illegal' oversized prims floating around. (You can find them on slexchange for instance), but LL frowns upon their usage. Although lately LL has allowed their usage on private island sims if the owner allows them (at your own risk), as far as i know.

If you mean a circular building with vertical walls (big cylinder), it's possible to combine hollowed,cut and flattened cylinders into a large circular area. The rotations, cut and alignment is tricky though, and as far as i know, there are no scipted 'generators' for such a structure.

Thanks
Yes I know about the old, oversized prims that are still in circlation here and ther, but didnt want to resort to "illegal" stuff.
And regarding the building:
Yes, the structure shold be like a srt of tower. Round walls on a circular floor. Diameter 20 or 30m (I want to use it with a panorma 360° picture wallpaper as an internal texture. With the 10m version you are to close to the picture for a nice effect.)

Cadro Murphy makes a tool to make this very easy.
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I made a flying saucer useing his tools.

Cadroe's tools are very useful for creating rings and spheres. They use 'box' prims to create the structure though. If anybody knows of a tool that does the math to create a ring/cylinder using hollow/cut/flattened cylinders i would be highly interested

There's an excellent tutorial in The Ivory Tower Library of Prim to make a 30m max round floor. I think it's on floor 3 of the library.

You can find landmarks to it at the New Citizens Plaza.

Thanks Thygrrr
I checked the Tutorial. And yes, your right, but:
They solve the problem by using pie shaped, square prims. so the result isnt realy round, its a box with x-amount of corners, all aproaching 180°.... the more you use, the nearer to 180° you get. But it also meens your using a lot of prims. And then try to set it up on some ones First Land.....
But your right in one point: Ivory Tower (based in the "Natoma" sim at 170, 180) is a must ! I go there again and again...

Since ShapeGen allows modification of its source code, replace the boxes it uses with the shape/shapes you want to rez, then use as per normal. I hacked up a version to work with giant prims in this fashion.

Isn't at 20 meters diameter the actual "jaggedness" of such a multigon close to the pixel boundary? I.e. could you even tell it's not a client-side tessellated prim (i.e. a cylinder)?

I've built 20M diameter towers that look like they are perfectly circular. I used 12 flattened cylanders, hollowed to 85% and cut at both ends. Each segment was rotated an additional 30 degrees, and moved into place. I used a building grid texture on some flat prims to lay out the wall sections on the correct positions, and was done with the basics in 15 min or less. The result, on the exterior walls, is a beautiful circle, with no visible seams. It even textured quite well, using 4 repeats per surface horizontally.

The down side was that on the inside, the cut ends angle toward the center of the eliptical cylander parts, making a v-shaped open wedge at each joint. I placed a cylindrical column over each joint, and was very pleased with the result. I think I can probably play with the cut begin and end, and use more segments, to minimize or eliminate that joint.

A detailed discussion on techniques can be found here. Look for Chosen Few's description of his method. It's the one I used.

Chosen Few also just posted a synopsis of his method for making huge cylanders in this thread.