The variables are VB6 strings, so in theory the limit is 2Gb, but real limits will definitely depend on memory and many other factors. The bigger the strings the slower they may be to handle and remember that some characters have special meaning in RR so if you use some special characters you may not be able to use certain functions with the variable.
If you use the plugin specific commands to get/set user variables, you don't have to worry about special characters - it's only when trying to use the variables in commands (i.e. "COMMAND;$VAR$") that you should worry about things like ||, <<, >>, ++, etc.
I'm not sure history.txt is the ideal place to look for changes to the SDK (even if they do rarely occur) SDK documentation should exist in a separate entity, preferably with each addition being added to the extension plugin examples. The idea is to make it easier to open up VBS and code right?