Quick look at sources show that

1. isc_event_block\gds__event_block and isc_event_counts\gds__event_counts make assumption that event name length is 1-byte wide,
i.e. it encode\decode event into\from event blocks using 1-byte length for names.
This give us up to 255 bytes for name string.

2. gds__event_block_a\isc_event_block_a assume that event names is no more than 31.
I.e. if application uses these API, it will be limited by 31 bytes names.
Comments shows that XXX_a API's is used by ADA preprocessor.

3. Event manager uses 2-byte for name length.

4. Engine make no guess about event name length, at least i see no such code.

So, i'd say you may use events with names up to 255 bytes (assume you not using XXX_a API's).
But, again, it was a quick look and i could miss something.

Regards,
Vlad