Funny thing that PSI doesn't state what size collets come with the set. In one of the customer question/answer I learned that it is an ER-32 set, which is very common and easy to get other collets for.
Jeff; When you get the collets, get a metric set of 18 as opposed to a fractional set. the fractional set has some gaps in coverage above 1/2". The metric set covers the full range of sizes.