left luggage/storage

Here is a picture of lockers in Fuessen Bhf. In my experience they are pretty much standard size throughout Germany. I think even the behemoth bag you describe, while it won't fit in any airline sizing box for carryon, will fit into the largest locker. I don't think the length will be a problem, but the 19" width might be. The coin/key mechanism takes up some width at the opening. You better hope there is a bottom locker available so you can roll it in. At 10# empty it's probably hard to lift full. Looking at the station's plan on the Bahn website, It looks like in Frankfurt Hbf the lockers are in a room to the right side as you come in the main entrance, along with a check room and lost and found. There are also lockers in the hallway between the bookstore and the Reiseburo, opposite track 19. The only lockers I see in Mainz are downstairs, under the main entrance or at the other side of the station by the Mombacher Strasse exit beyond track 8. I don't see a check room. According to the German Rail website, Bahn.de, there is no such station as Lutherhaus. There is a station called Lutherstadt. According to German Rail website, Lutherstadt has some form of baggage check, probably lockers.

Charmz, I believe the largest locker will accomodate your large wheeled suitcase. It's pretty big in length, width and depth. That large one costs 5 Euro for a duration of 72 hrs. All three cities you listed have coin locker facilities, whether Lutherstadt Wittenberg also has the largest lockers like those in Frankfurt Hbf I don't know.

The arrangement of lockers does vary slightly throughout the country. Most have a few different sizes, but some of the stations (like Köln), have a different system where you place your bag in a sort of dumb waiter, and it gets stored underground. As long as you're not traveling with steamer trunks, you shouldn't have much of a problem fitting your luggage.