Note that under ATE, by default, the maximum length of setdef is 3072 bytes. Beyond that, you will arguably be better off using a control such as XTREE. But if you can expand that limit by executing the following command:
PRINT TAB(-10,42);BUFSIZ;chr(127);
where BUFSIZ is the new maximum length of the ATE command buffer. The buffer must be large enough to handle all of the arguments of INFLD, which for the purposes of combo boxes, mainly just means the maximum size of setdef, plus perhaps another 1K to handle the rest of the parameters.
The maximum size of a setdef match is 300 characters.