Many Anchor Stone fans say that you need a set #6, or even #8 (6 + 6A) to build anything of interest.
But I rather enjoy building with smaller sets, even the smallest - set 0, as shown above with some battered older stones.
The current maker doesn't offer sets smaller than a #4, but plans for the smaller sets can be found online or acquired through ankerstein.org.
This smallest set would make a nice gift to potentially entice friends. The current offerings are too expensive for any but the sincerely interested.