The character limitation for letters is 7500 characters, spaces included.
There are two options to consider: (1) diminishing the amount of text so the letter is 7500 characters or less, or (2) split the letter into two files.
If the letter is split into two files and a patient signature can be electronically attached to one file, the office may consider the inclusion of a signature on the other file. That way, with both files signed, the office can assume patient acceptance of both files.