A single Union structure is limited to 16 MB (16,777,216 bytes). Locally dimensioned Union structures are limited to the amount of free stack space available, typically less than 1 MB. If larger UDT structures are required, use a STATIC, GLOBAL, or INSTANCE declaration instead, since these are not created on the stack. The same rules apply to User-Defined Types (and LOCAL fixed-length and nul-terminated strings). An individual array within a Union may occupy as much as the full 16 MB Union size limit.
User-Defined Types (UDTs)