So my solution to my own problem: After playing around with redis-cli a bit longer I found out that DEBUG OBJECT <key> reveals something like the serializedlength of key, which was in fact something I was looking for…
For a whole database you need to aggregate all values for KEYS * which shouldn’t be too difficult with a scripting language of your choice…
The bad thing is that redis.io doesn’t really have a lot of information about DEBUG OBJECT.