There's no best and worst table runner size. It's really up to how much space you have and personal preference.
The most popular widths thus far have been 24in and 30in.
There are no standard sizes at convention centers - heights, depths, and widths can vary greatly.
I offer table runners at 72in long - which is a great size to flow from the floor on the front of a table, across the table, and down the back. However, I can offer longer runners if that's something that's necessary.
For the width of runners, keep in mind how much space you have.
If you're splitting a 6ft table in artist alley, you don't want a runner wider than 3ft.