I can set and get a token (VTL type, like in your example) of well over 7K using the API. Just tested with a 100K payload and it's fine.
Suspect your client library is mangling the payload.
Also, this doesn't have a heckuva lot of inherent meaning:
You can't be HTML-encoding ampersands in URLs on the wire. So this may be debugging output from your client app but it doesn't reflect (I certainly hope!) what it's sending to Marketo.