I have a 10cm Glossepsis wanamensis.
My forays into keeping larger Geophagines resulted in a lot of space being used for large tanks, and endless water changes. Most people who go for big fish don't have the huge tanks that are needed to do it right, but they are usually pretty confident about doing it wrong! My largest tank is a 120 gallon now, so my largest fish is a rainbow (he has a bunch of almost equal sized companions).
The worst I ever saw was a wise looking old giant gourami that when side on to you, had 10cm on either side. He lived for years in a large cube, but moved in tight circles.