Wow, that is a very high resolution for such a large image! Please post a screenshot of your export dialog where you select these dimensions? What is the size in pixels of the original image you are trying to export. If your original is a different aspect ratio than what you want to end up with, it may not work as you expect.
I am trying to export a tiff file to 204" x 96" at 300dpi. Lightroom exports but the size only goes to about 96 inches. Is there a limitation in LR?
The exported TIFF would be 61,200 pixels x 28,800 pixels:
204 in * 300 pixels/in = 61,200 pixels
96 in * 300 pixels/in = 28,800 pixels
The image would have a total of 1680 megapixels:
61,200 pixels * 28,800 pixels / (1024 * 1024) = 1680 megapixels
Unfortunately, LR only supports images up to 512 megapixels (import and export).
But as JoeKostoss suggests, do you really want such a huge image at 300 dpi?
Client request for the file, I know it will not be printed at that res, but they want it anyway. Working from an 35mb raw file, aspect ratio is the same.
Also, the exported image would be too big for TIFFs, which have a maximum file size of 4 gigabytes:
8-bit: 61,200 pixels * 28,800 pixels * 3 bytes/pixel = 5.3 GB
16-bit: 61,200 pixels * 28,800 pixels * 6 bytes/pixel = 10.6 GB
You'd have to edit the image in Photoshop, resize it to the required pixel dimensions, and then save it as a PSB.