If you right-click on the details pane in either the "Variables" view or the "Expressions" view, there is a Max Length... menu option. You can enter a number there to configure the truncation of string output. Keep in mind that the calculated length includes the type identifier at the beginning, so str: 12345 is actually 10 characters, not 5.
There seems to be a hard limit of 1000 characters, even when entering 0, which the field description claims means "unlimited," so no matter what you put in, you will never see more than 1000 characters. I don't know if that's a known issue or not; I have not looked into it. For strings or other variables longer than this, I generally use the logging module from the standard library and send the string to a log file or stderr.