At the time of writing, as per How Splunk licensing works :
Unlike event data, metrics data counts
against a license at a fixed 150 bytes
per metric event. Metrics data does
not have a separate license. Ingesting
metrics data draws from the same
license quota as event data.
It's a fixed 150 bytes at this stage...
It means that each metric event, regardless of its content, will count as 150 bytes against your daily indexing license allocation. A metric event with no dimensions (just time, measurement, and value) will count as 150 bytes, and a metric event with 15, 20, 25, 30, etc dimensions will also count as 150 bytes.