How long is a Bitcoin? I searched already but I couldn't find anything about the structure of a bitcoin.

My understanding is that a bitcoin, which is different AFAIK from a bitcoin address, can be interpreted as a string of numbers and characters (alphanumeric). How long is a bitcoin string (min/max)?

I found here (https://blockexplorer.com/block/000000000000000002109f32aeac7a036232b0116f939326651e739a28e6d6b6) now a size of: 934325 bytes for a particular bitcoin. Could this be the length of the particular bitcoin?

PS: I searched always for length and not for size...
A bitcoin is not a piece of data, and therefore does not have a length.  A bitcoin, the unit of currency, is just like any other unit of measurement.  It wouldn't make sense to ask "how much data is a meter?", and likewise it doesn't make sense to ask how long a bitcoin is.

That said, the amount of bitcoin that is stored by an individual is the sum of all the bitcoin denoted in the unspent transaction outputs (UTXOs) that are controlled by the individual.  These are pieces of data and do have a size...albeit a variable one.  These outputs do not need to be stored by the owner, as they are stored on the blockchain.  All the owner needs to store is private keys that allow them to spend the UTXOs, which is what a wallet manages for you.

I hope that helps.