For a small, new division you should use a medium sized pot, often this is 9cm.
A Venus Flytrap does not produce a huge amount of roots, however those that it does produce must have plenty of room.
Should your division start separating, or you notice lots more leaves growing away from the central crown of the rhizome you should either divide the new crown and replant, or re-pot the entire fused rhizome in a bigger pot.