Summary
Question
Its not clear the maximum size of the global lists, there are some information "https://support.microfocus.com/kb/kmdoc.php?id=KM41790" that points to no more than 1000 entries.
Also some forum with more information: https://community.microfocus.com/t5/Service-Manager-Service-Center/Maximum-of-1000-entries-in-list-for-file-assignment/td-p/1049396
But still no clear data about it, a user could runs GL with more than those limits and still works as normal.
Answer
The maximum size of a globallist is limited by the max size of the globallist record which is 2GB. Both the Display list and Value list are stored in the same GL record and these values are stored in the RDBMS as a BLOB so that is the limiting factor.
Most GL's are used in form combo box drop down lists. Generally for both UI performance and usabilitiy a drop down list of 1000 or more entries is not recommended, however we do not impose a limit. The larger the list the more memory will be used by the client/browser.
If they really need a huge GL then the limit would be 2GB/(number of entries*size of each entry). If the GL has both a display and value list they would need to take into account i.e. the number of entries would include both lists.
For example a GL with a Display List and Value list where each element is 10 bytes
2147483648/(10*100,000,000*2) approx 2GB. So in theory you could have a list with 100,000,000 elements but if you tried to use that in a combo box the browser would hang and the performance impact of a RAD variable with 100,000,000 elements would be huge.
For that reason the rule of thumb is to limit a list to around 1000. I have seen customers with lists of 5,000 and not have problems, but again that is not a guarantee, as each system is different.