In the photos of my work you can see the contraption that I use as a rigging. It's made out of the metal strut that is used for hanging and bracing pipes, the most common brand name for which is "Kindorf." With a little welding and machining I've got a $200 version (excluding labor) of a $2000 easel that can hold a painting from 12 inches square to 12 feet. The only actual limitation is the size of the room. (I've removed the photographs that were here. Follow the link below)
The vertical members each roll on two ball bearings. A small piece can be clamped in place at any height and a larger one simply hangs on two movable posts that click into the verticals. I've been told I should market it, but I'm too lazy. I'll add more detailed instructions later. The clear bracing is not plexi, which is too fragile, but "Lexan." The toggle clamps are from McMaster-Carr. See the link under "misc(ing)" at the bottom of the page.

Here's a full page of images: "Kindorf" Easel