What size billiard balls are appropriate for a six foot billiard table?

Pool balls are standard size regardless of the size of the playing surface of the table. Six feet is on the extreme small size of pool tables. Pockets and rails also fall within standardized ranges, so jumping balls over the rails and pocketing balls shouldn't be a concern. Congestion is just the nature of the beast the smaller the playing surface is. Now if your table isn't regulation, I can't be sure you won't have issues.