[Keith Teneyck]

Open your image up in Photoshop. Convert it to 600DPI divide its size by three.

I had an image so big (48000px X 2160px @72dpi) it was saved as a .psb (Had the same issue)

I then divided 48000×2160 by 3 and saved as 600dpi

Open up in AE. Create new comp (in my case 3840×2160)
Drag imported 600dpi precomp into new COMP, scale it three times the size of the comp you need.

Output it as you would. BOOM.