The size limit is 32767 x 32767 pixels. The other limit is 2 GB of memory. PSP is a 32 bit application, and no matter how much memory your computer has or your operating system can use, PSP can only address 2 GB of it.
Am I right in guessing that your image has more than one layer?
Another problem would be that while the resizing is being processed, PSP has a copy of both images in memory.
Your original image is 4800 x 7200 pixels. Assuming that it is a 24bit (or 8 bits per channel) image, each pixel needs three bytes... one each for red, green and blue. That is approximately 100 MB.
The new image is 7200 x 10800 pixels and will take up about 220 MB.
While the resize is being processed PSP will be using 320 MB on top of the amount that it uses for itself... maybe around 70 MB. (Maybe more if there is a lot of stored undo data).
Each additional layer needs 4 bytes per pixel. The additional byte is for opacity. So your original image uses about 230 MB and the larger size uses 520 MB. While the resize is happening PSP will be using over 800 MB.
Another layer or two, and PSP will be exceeding the 2GB that is its maximum available memory.