Thanks for reaching out to us!
These dimensions are unfortunately larger than Ribbet currently supports for printing in high quality. You can select the 24 x 18″ preset in the constraints drop-down (on the left bar when creating your collage) as this is in the same proportions as 48 x 36. If you select HQ (on the top toolbar) this will allow printing at 48 x 36″ at 75 DPI. Normally for high quality printing you’ll want at least 150 DPI, but at 75 DPI it’s possible it will be acceptable.
Regards – The Ribbet Team