5120 seems to be a reasonable choice because, firstly, it is divisible by 1024 ^{otherwise RAM might be used  inefficiently} and secondly it is just slightly more than 1/4 RAM assuming 16 GB leaving enough memory for the OS and other background processes.

5120 seems to be reasonable size as firstly it is  divisible by 1024 ^{otherwise RAM might be used  inefficiently} and secondly it is slightly more than 1/4 RAM which left the space for other OS operation. But if the system is not very busy in performing other task dbcache might be increased to 1/2 or even 3/4 RAM,i.e dbcache=8182 or dbcache=12288. However, if the system isn't heavily used for other tasks it's safe to allocate more memory to dbcache. In such cases, increasing it to 1/2 ^{dbcache = 8192} or even 3/4 ^{dbcache = 12288} of  RAM can improve performance.