(my original post seems to have disappeared so I'm re-posting).
The official Deutsche Bahn website ( https://www.bahnhof.de/koblenz-hbf/schliessfaecher ) tells me that there are good sized storage lockers in the Koblenz station; two sizes (24 × 42 × 76 cm and 44 × 61 × 91 cm, suitable for large cases), well priced.
So the only question is, are they likely to be available at around noon time on a Monday in October (Oct 6)? I do see that there are additional lockers at the nearby Forum Mittelrhein location (https://www.forum-mittelrhein.com/services/schliessfaecher-s17089/?location=p480-10008 ), though they don't give dimensions, only saying 'suitable for rucksacks and handbags'. I'm thinking 'rucksacks' can be pretty big!
We had great success using the lockers in Zurich train station a few months ago, so we really appreciate this convenience.