With the informCacheSize setting you can configure the amount of SNMP informs that will be stored in a cache per SNMP enitity (source IP:source port).

DataMiner will only store the inform ID (but will do that twice; once for fast lookups and once to know which one was inserted first and needs to be popped when the cache is full) so the memory footprint of a larger cache will be minor. As a rough calculation, you could consider maximum 40 bytes per inform so for a cache storing 2000 items that would be 80 kB per element receiving informs.

In terms of checking if there's a cache hit, a larger cache of around 2000 items should still be fast because a hash code of the inform ID is stored in a hash table for fast lookups.

So while the impact will be rather insignificant, it's not recommended to configure it too high neither. If the data source is still sending out retries for unacknowledged informs that were sent out 2000 informs ago, you might be masking an issue that could potentially better be solved on network level.