We routinely shoot paintings in the 4' x 7' range by shooting them in sections in our normal copy set-up. Not-so-routinely, we shoot up to 7' x 12' canvases, also in sections, although this requires the setting up of our "monster" wall. .

The theory behind shooting "panels" of large works of art is simple enough. You divide a canvas mathmatically into sections that your camera's sensor can capture at 300 dpi ( 240 is fine for canvas, no matter what anyone tells you to the contrary*), shoot the sections and align and stitch them together in Photoshop and you've now generated a digital file that can reproduce the work full-sized on canvas or paper. But how do you keep the sections square (not key-stoning) and uniformly lit? How avoid the headache of having to match the "panels" with endless micro rotations and transformations? Simple, so long as you keep in mind some basic geometry and physics.

Linearity: maintianing three perpendicular axis without tearing your hair out. .
It is essential that each section be precisely square in all three planes to the axis of the lens. In our copy studio, we have an easel attached to a ball bearing tram (an industrial door roller) that is attached to an overhead beam. The easel maintains plumb (perpendicular to the floor) and tracks smoothly left to right for a distance of ten feet. The camera is on a roller dolly, the tracks of which are exactly perpendicular to the easel's path. (the reason for the dolly is for re-framing paintinigs of different sizes as the shoot progresses, without having to re-square the lens axis to the easel) At the beginning of a shoot, the camera's lens height is exactly the same as the exact center of the easel, which gives us a perfect horizontal angle. We have our x, y, and z axis all at right angles when we begin, so the problems of keystoning (creating non-rectilinear panels) has been eliminated.. Also, because we shoot with a relatively long lens (380mm APO RONAR), there are never issues with lens barreling or distortion.

Lighting
As for lighting, it's no big trick to lighting a 3' x 4' area uniformly (my max size with the super 6k back), so I won't spend time on this, but to say that it's essential to your workflow, as if you have to make minute exposure adjustments in photoshop, you start throwing things off. Again, because of the length of the lens we use, side flares (which would kill you in this endeavor) and the need for the band-aid of cross-polarization is eliminated from the get-go.

Shooting and assembling Considerations
The sections should be shot with an overlap, 10% at least, the more the better. When you assemble the pieces in photoshop, we've found it's best to use individual layer masks and paint the "stich" with a feathered brush creating a circuitous path, usually through solid areas if possible. We of course make the active layer temporarily semi-transparent so we can see what we're doing. The panels always line up horizontally because of the way they're shot on the rolling easel, so this is never an issue. If there is a minor horizontal "roll" problem, (never more than 1 degree), we stitch the painting together first, then rotate the assembly. If there is ever any small exposure variation, left to right or up to down, we use a dupe adjustment layer in the "screen" blending mode, using layer masks and gradients to fix the exposure. This has no color shifts resulting as would levels, curves, etc.

REALLY BIG PAINTINGS
We have built a rig (our "monster wall") which we can either use in our studio or travel with for shooting big canvases in, say a museum or a corporate lobby. It incorporates all the above principles, but works from the ground up. It is basically a precisely made rolling "easel" wall. which is dead flat and plumb. It rolls on a skateboard wheel dolly on ABS pipe which all film makers are familiar with. This pipe, it is noted, can be bought on a distant location for a couple of bucks and tossed in the dumpster at the end of the job. As a picture is worth many words (depending on the language and the photographer), here's what we've used successfully. Really good carpentry is essential.