I think I understand your problem, now. The Data (with a capital D) is a two-dimensional array of some size (say, 10 rows and 3 columns) and some data type (say a 4-byte integer). If you ask how many bytes does this Data take in memory, the answer is 10*3*4 = 120 bytes.
The problem comes when you try to write out this data to a file, and ask "How many bytes are there in the file?". The answer depends on several things, but one very significant one is what kind of file is it? In your example, your data seem to be read from a text file, so the numbers are represented as strings. Thus a file with 100 values of 1 would be much smaller than a file with 100 values of 1000000, as you'd only need a single character to represent the number "1", but would need 7 characters to represent a million. You are using the Spreadsheet String to Array function, which may have additional "padding" rules, but the bottom line is that text representation of numeric data need not bear any fixed relationship (in size) to the number of data elements.
If you read and write your data using the "binary" read and write functions, you should create a file of 120 bytes (in the example above), and could easily pick out a single element from the file using the file positioning function. It is also likely that the file will be smaller, and file i/o faster, though you'd absolutely need to know the format of the file to make sense out of it (for example, if you tried to read it assuming it consisted of DBLs instead of I32, you'd get values, alright, but they wouldn't make much sense!).
