we have booked first class where do you store your suitcases. ?
No difference between first and second class for luggage. It is your responsibility to carry your own luggage through the station, on and off the train and store it in the coach you are travelling in. The only difference in 1st class is here are fewer passengers per coach, so slightly more space per person.
For more info on this (not specific to Italian trains) see here: https://www.seat61.com/luggage-on-european-trains.htm
I found the website for the "Cassa alla Fenice" here: http://www.casaallafenice.com/en/
According to this website:
From Piazzale Roma or from the Railway Station
From Railway station or Piazzale Roma you have to catch water-bus line N 1, direction to S. Mark, Rialto and get off in S. Angelo stop. It takes you 20 minutes.
Upon disembarking form water-bus in S. Angelo, go straight ahead and turn in left , passing in front of Hotel Corte dell’ Alboro. You will find a small bridge, cross it and turn right after it; go straight on and turn left entering in Calle della Mandola. Follow this street until you’ll see a window with books in the corner , turn right and go ahead (you are already in Rio Terrà degli Assassini). After 100 meters, just before the canal, you will find on your left side The entrance of Locanda alla Fenice.