I've had the same question before, and after doing a LOT of reading on the topic, I have the following advice: Don't do it.
Fish growth is unpredictable except by the most dedicated scientists willing to control every aspect of the environment and perform repeated measurements and very careful readings of daily food intake. What it boils down to for most of us is that fish will grow. Fast, slow, medium, all of the above at different times, but they will grow.
There is no way to predict when a 55 would be too small.
Grizz is on the right track. There have been some great posts in here about fish growth in small tanks, but I'll sum it up.
Kept in too small a tank, even a juvenile fish can begin to experience severe skeletal deformities, organ failure, and premature death. If an adult fish needs X amount of space, then a juvenile of the species requires the same amount.
One final thought, even if someone chooses to ignore this and just say "I'll trade him when he grows 2 inches", we see our fish every single day. The only way I realized how big my clownfish had gotten was by adding a mate for her and suddenly realizing she had doubled in size.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef