As a rule of thumb how tall can a load be before it requires a step deck trailer ?

It depends on the deck height of the flat bed. Height can vary due to tire size, etc., Some states require permits if you are over 13'6", others will allow 14'. There is no real rule of thumb, but if the total height is more than 13'6" then it would probably be best to put the load on a step deck, double drop or RGN. In most cases you should be able to put something on your flat bed which is approximately 8' to 8'6" tall without having to worry about being over height.

what is a RGN like a lowboy trailer??? What does it stand for.

RGN stands for removable goose neck. Most newer lowboys are RGN's but there are still lowboys that rear load only.