The queue size doesn't affect performance in general. It determines the maximum number of packets in the rule's queue before the rule begins to drop packets. Since your server has enough RAM you don't need to change the queue size. The min. free RAM size in bytes for stable operation can be computed as MaxQueueSize * NumberOfRules * MaxPacketSize, so in your case 500 (default) * 150 * 1600 = ~114 MB.