What Domino Sizes Are Available?
Dominoes are available in various sizes, including the following:
Mini Dominoes - About 1-3/16" long by 9/16" wide by 3/16" thick
Standard Dominoes - About 1-7/8" long by 15/16" wide by 1/4" thick
Professional Dominoes - About 2" long by 1" wide by 3/8" thick
Jumbo Dominoes - About 2" long by 1" wide by 1/2" thick
Tournament Dominoes - 2-3/16" long by 1-3/32" wide by 1/2" thick
In addition, there are also a few "giant" dominoes avaiable, though the sizing is not standardized.
So why choose one size over another? It comes down to what games you want to be able to play, and how much space you have available (i.e. how big your table is).
If you're playing on a small surface, smaller dominoes would be the best choice. If you have more space, the larger tiles are easier to handle, as well as being easier to read.
Some games require you to hold the dominoes in your hand, as if they were playing cards. In this case, the limiting factor is your manual dexterity - how many tile you can comfortably hold before you start dropping some?
In other games, you place the tiles on the table before you, laying on their side. In this case, a thicker tile is better, as it is less likely to topple over if the table is bumped.
So the question as to which is best comes down (as most things do) to your own personal preference. If you're unsure, try a few different sizes until you find one that feels right and fits in with your playing style.