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In a PLC-5, an "array" is defined by a Data Table file. These have data types (the most common is a 16-bit signed integer) and Data Table File Numbers.
Because the data tables are user-defined for fixed sizes, and because of the age and sophistication of the operating system, there is no mechanism for the user program to read their size. You need to write your logic carefully in order to avoid exceeding a data table border.
The RSLogix 5 software won't let you directly address an out-of-range Data Table element, and when offline should let you expand the data table to accommodate an entered address.
The maximum numbers of elements/words per data table file are well summarized in the 1785-QR001 quick reference.