If the argument to SIZEOF is a parenthesized expression, the result of SIZEOF is an integer value that is the size of the type associated with the expression. The assembler reports an error if the expression has no type.
If the argument to SIZEOF is a section name, the result of SIZEOF is a link-time expression representing the size of the given section. It is an error if the section name is not known to the assembler.
X VAR LONG XSIZE EQU SIZEOF X ; 400, 100 four byte elements X0SIZE EQU SIZEOF X ; 4, size of LONG