To find the record length, look at the record layout described by the FD entry in the File Section of the COBOL program that uses the data file. The record length is the sum of all the lengths of all the elementary data items which make up the FD group item.
Each character position occupies one byte for PIC X and PIC 9 items. To find out how to calculate the length of computational fields (COMP-1, COMP-2, COMP-3, COMP-4, COMP-5 and COMP-X) see Selection of Character Representation and Radix.