Sherry pie just gave you the conventions - well, the explicit game rules actually.
I based mine like
GW did for Epic 40,000; three Warriors or one Lictor on a standard Epic “strip” base trimmed town to 3/5 its usual length (i.e. cut off the two outer holes).
Alternatively, the 2nd Edition Space Marine method - 3 Warriors or one Lictor on a 20mm square base would also be appropriate.
If you’ve got loads of Warriors you might otherwise never use, you could put seven Warriors on a 40mm base to make a mighty swarm.
Termagants and Genestealers on standard 40x10(ish) bases, Lictor and Warriors on cut down 24ish by 10ish bases, Hive Tyrant on a 20mm square, Dominatrix on a 50x25mm base and a Heirophant on a 70mm round base (i.e. one just big enough for it to fit)