Half dollars are a great size to facilitate a lot of coin moves. if you have huge hands (doubltful) dollar coins are good. It also depends on the trick you're doing, and the venue you're doing it in. But i generally use half dollar sized coins. They are the same size as english pennies so you're set for copper silver routines. Barber half dollars (same size as kennedy halves) are really good if you are doing any routines where the coins are sliding around eachother secretly because barbers a usually pretty worn down and don't make much sound when they slide over eachother. They're also quite pretty. But not necessary at all. I've never owned any walking liberty halves so i can't really say much about them other than that they're really pretty. I don't know if you can find them soft or not (soft=smooth. oh and soft coins are not a must have as that guy with the long name said).
Just go to the bank and pick up five kennedy half dollars and five eisenhower dollars and you'll be set for most coin routins you'll come across. Then you can just pick up whatever else you need as you need it. So if you come across a copper silver routine you want to do then you can just hit up a coin shop for some old english pennies (which are like 50 cents a pop) etc.