Q:
We know there is restriction for the maximum PDF length/dimensions is 5 meters. Will PDFTron allow to write a PDF longer than 5meters?
A:
PDFNet does not enforce any page size limits, though other libraries may. Keep in mind that going beyond the 14,400 limit may break PDF/A-1 compliance, in case that happens to be a restriction for you.
Also, note that starting with PDF 1.6 you can change the Page the page default user space (default is 1/72 inch per unit) to anything else, such as 1/36 inch, thus doubling the size of the page if printed.
See Page.SetUserUnitSize() method. This way you can stay within the 14,400 unit limit, but go beyond 5 meters when printing/rasterizing.