I'm not a particular fan of odk:length either. My preference would be that the client - Collect, Enketo, ... - should simply send the entire result [its not like XML have any restrictions on its length...] and its up to the backend - Aggregate, Central, ... - to make sure it saves the entire result, in whatever internal format (string, blob, what-have-you) is most appropriate to whatever database it happens to be configured to use (Postgresql, MySQL, ...).
<soapbox>As much as possible I think this internal default string size should be completely hidden from the user, including the form writer. If that means storing potentially arbitrarily long geotraces/geoshapes in a different database format than geopoint, then so be it.
</soapbox> [or perhaps this is a more appropriate discussion to have over in Slack?]