I order to avoid absolutely horrible performance, SlickEdit will break long lines up multiple lines of about 4k each (Tools>Options>File Options>Load> Wrap line length). I wouldn't recommend making this much large since the display performance will degrade quickly. The color coding doesn't support correctly color coding this kind of multi-line string yet.
The code for chosing where to break the line is very tricky. I'll see if there's a way to make the lines break in more even chunks for this case.
When I really need to view long lines without them being broken up, I give SlickEdit the sunny day scenario. I make the file a .txt file (no color coding), turn of softwrap, make sure I'm using a fixed font, and if necessary open the file as SBCS/DBCS (definitely needed for Mac and Linux to override default Utf-8 encoding). Seems like we could use an automated way to open a file in the sunny day scenario. This won't work if the file contains Unicode characters.