PortraitPro since version 15.4 has an upper limit of 500 megapixels for 8 bits per color sample, and 250 megapixels for 16 bits per sample. Note you also need plenty of RAM (4GB or more).
Note that the size limit only depends on the number of pixels in the image and not at all on the size of the file when stored on a disk. This is because the picture needs to be decoded into memory and then processed by PortraitPro at that size in memory. For example if you have a picture that is 7000 pixels wide and 4000 pixels high, then this is 7000x4000 pixels which is 28 million pixels, i.e. 28 megapixels.