Fastest way to get Google Storage bucket size?

Unfortunately, no. If you need to know what size the bucket is right now, there's no faster way than what you're doing.

If you need to check on this regularly, you can enable bucket logging. Google Cloud Storage will generate a daily storage log that you can use to check the size of the bucket. If that would be useful, you can read more about it here: https://cloud.google.com/storage/docs/accesslogs#delivery

The visibility for google storage here is pretty shitty

The fastest way is actually to pull the stackdriver metrics and look at the total size in bytes:

Unfortunately there is practically no filtering you can do in stackdriver. You can't wildcard the bucket name and the almost useless bucket resource labels are NOT aggregate-able in stack driver metrics

Also this is bucket level only- not prefixes

The SD metrics are updated daily so unless you can wait a day you cant use this to get the current size right now

UPDATE: Stack Driver metrics now support user metadata labels so you can label your GCS buckets and aggregate those metrics by custom labels you apply.