Hello Eric, the biggest or wides keys are on the Hohner Claviola. Rare with the sound of a clarinet. The next would be The Eolina longer but about the same width as the the Hohner professional 36. As mentioned by Alan, the Hohner pro 36 in a good condition you should expect to pay $350-$400 one in prestine $400-$500, but a bargain compared to an Eolina about $2,700-$3,500 depending on condition. The Claviola like the Eolina anywhere from $1,800 to $2,500. Be sure to ask a lot of questions on these models as they are no longer made and repairs will be a little costly unless you can make these repairs yourself. Most Melodica, Yamaha, Suzuki, Hohner, have the same key size (excluding the mini keys) if you are looking for a Melodica with keys the same size as a piano, I don’t believe you will find one unless it is a one off or custom made.
Hope this helps