This shows the size of data stored on the L2ARC cache devices. This is expected to increase in size over a period of hours or days, until the amount of amount of constant L2ARC eligible data is cached, or the cache devices are full.
When troubleshooting L2ARC warmup. If the size is small, check that the workload applied should be populating the L2ARC using the statistic Cache: ARC evicted bytes broken down by L2ARC state, and use the Protocol breakdowns such as by size and by offset to confirm that the workload is of random I/O. Sequential I/O does not populate the L2ARC. Another statistic to check is Cache: L2ARC errors.
The L2ARC size does shrink, if data that was cached is deleted from the filesystem.