Baskets each hold two 3-gallon buckets.  They fit Arctic Air models ST13G (3), ST15G (4), and ST20G (5).  Baskets are available singly (part #612777900) or as a pack of four (part #216444900)

Specs:
Length: 19.5”
Width:   10.5”
Height:  8”
Weight:  3 lbs. Each