The closest Ikea is in Godorf, but there are even bigger ones north of Cologne and in Koblenz.
Porta is probably the largest furniture store in the area. One is off the highway on the way to the Godorf Ikea and the other is the exit before the Bonn-Cologne airport.
There is a mambo in Bonn Center, but it has only household items, no furniture. The one in north Bonn is several floors high and has furniture for every room in the house - even kitchens.
High quality children's furniture. Their shop is about 45 minutes away from Bonn to the west of Cologne.