theory11 — Magic Tricks & the World's Finest Playing Cards

1-on-1 Coin Sizes

Kenners stuff used Morgan Dollars I think and Flicker used half dollars. Trifecta also used Morgan Dollars.

Tho you can do most of the routines with halves.
 
If I were to Suggest Any coin... I would use Eisenhower Silver Dollars, perfect size for me, and for almost anyone as well as pretty easy to find!
 
Kenner, Homer, and Doug all use larger coins such as Morgans, Kennedy, etc.

But that's only because their hands are bigger. If you have small hands, and have trouble palming a half dollar, then by all means use a quarter.
 
The size of their hands doesn't have as much to do with it as that the larger the coin the more visual the effect. Any of those routines that you mentioned can be done with quarters. However, smaller coins limit your distance from the audience and what they are able to see.
Also, Morgan silver dollars also have a really nice ring to them. Kennedy half dollars made before or around 1966 have some silver in them and ring rather well too. The sound just adds a little extra class to it.

It's really about preference and visibility.

Cheers!