i would like to send data to a satellite modem in binary format. So to successfully send the message, the modem has to know the length, in bytes, of the message the arduino is sending.
Are you planning to send the whole file in one package? That will require reading the whole file, and storing it in SRAM, of which you have only 2048 bytes (and the SD library needs 512+ of them). If so, simply count the characters as you read them from the file (you'll need to do that to store them in the correct place in the array, anyway).
If not, and this is the more likely scenario, then the size of the csv file is irrelevant. It is the size of the package that you need to know. And, that depends on how you want to package data to be sent. Maybe, one record at a time. Of course, this requires that you know what a record looks like (i.e. when it ends).
