Read all of the file in, as uint8, in binary format (not text), and use numels() to determine the size of the resulting array.
Possibly someone else might be tempted to respond with a possibility about using dir() and examining the file size, but if you are using Windows then the size indicated in the directory for a text file might not match the number of characters you can read from the file, even if the file does not contain any Unicode: On Windows, reading from a text file ends at an EOF (decimal 26, control-Z) character even if that is not where the directory thinks the file ends.
