PB bag sizes are a bit confusing when it comes to size and grade of plant, hopefully, the following will help you out. The size of the bag does not always correspond to the size of the plant. That's down to the variety of plant. PB corresponds to pint-sizes, so a PB3 bag holds 3 pints (or 1.5 litres) of soil mix. Our standard sizes are PB2, PB3, PB5 and 1.5L bags.