I think you've got your terms backwards. What you have is already a rasterized layer. Raster data is pixel-by-pixel information, and will pixelize when scaled. (How much it will pixelize is a matter of resolution and how big you scale it.)
It sounds like what you want is a vector layer. Draw an ellipse with the ellipse tool (same toolbar button as the rectangle tool). Make sure to change the option in the option bar at top to the pen so it makes a path, not a shape layer. You can Stroke the path with a line using the path tool, though be aware that strokes in Photoshop are rasterized. For better vector capabilities, use Illustrator.
Another option would be to draw two shape layers with the ellipse tool- a black ellipse and a white ellipse. Make the black one larger, link the two together, and set the white one as a clipping mask for the black one. (Or possibly the other way around, I always forget. Check the help files for info on clipping masks.)
Yet another option, if you have Photoshop CS2, would be to use a Smart Object. Make a really big ellipse, at least as big as the biggest size you need. Then make the layer a Smart Object, and scale as necessary.