Size of holes for salt/pepper/seasoning shakers?
I don't think there is any set standard for the size of the holes. Most of the salt and pepper shakers I've seen sold to be used at the stove or grill don't necessarily have larger holes, but they have a lot more holes. They also hold more salt or pepper than table sets. This is an example similar to most I've seen:



If you look at the vast amount of salt and pepper shakers available, you will see that they come with very few holes, lots of holes, different size holes, etc. I think the key in what to use is to find what you feel comfortable with. 

Also, consider what type(s) of salt and pepper you are using. Most shakers are sold for use with table salt and ground black pepper. But if you are using a more coarsely ground salt or pepper you would obviously need larger holes.

I, like most of the others who commented, don't use shakers while I'm cooking. The only time I would use shakers is perhaps when seasoning food before or after it is cooked. The real downside is that if you try to shake salt or pepper over hot foods while cooking, steam can cause the salt or pepper to clog the holes.