PoznaĆ's main train station has lockers for large luggage if you read this link, but not sure if there is a time limit on how long you can leave stuff:-
inyourpocket.com/poznan/main-train-station_17430v
If you are, have been, or will be staying at a hotel and not just passing through, maybe you could ask if the hotel could keep your luggage for an extended period of time? Obviously they will charge for this, but I wouldn't have thought it would be that expensive.
There's also possible that you'll find something on the main bus station. But try the lockers at the train station first, I'm not sure how "large" they are - some mentioned on the other site that it may be a problem with bigger luggage.