The extra large yarn bowl is the most versitile size.  At 8" wide by 5" tall, it is big enough to hold multiple skeins and the fiber needed for large projects.  This is the 'blanket size' and is the most popular with the serious fiber artists.
How do we know this? The serious fiber artists and fiber sellers bee-line for this size and their eyes glaze over with glee when they pick it up and daydream of making it part of their collection.