Virtual Memory size is the amount of "address space" in use by the process. See https://en.wikipedia.org/wiki/Virtual_address_space. It's effectively just memory address ranges that are allocated but don't necessarily take up any physical RAM.
Real Memory size refers to resident memory size (or resident set size). This is the actual amount of physical RAM being used by the process.
Database Size on Disk refers to the sum of the size of all dbspaces including the temporary dbspace but not the transaction log.
Tools will depend on OS. Beware the task manager for Windows which, IIRC, shows values like "Memory Size" and doesn't tell you exactly what it means. Perfmon on Windows was much clearer about the various memory metrics. For Linux, there are countless tools (top, /proc contents, ps, ...). Google for other tools Windows, Linux or whatever OS you are using.
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