I'm not aware of any limitation in the SYLT format.
For the plugin, I tried 1000 chars and all is fine. When I tried to double the line length, I just hit a wall: max line length seems to be 1024 chars...
And, as I thought, the maximum length of whole lyrics the plugin is able to handle is 65536 chars, the max size of a string in VB...
The first limit you'll have is the display. You'll have to decrease font size to see that much: at font size 8, I'm able to see 210 chars. In 6, there's about 320 chars visible on screen (at 1280*1024), but I'm not able to read... If you plan to use these lyrics on emphatic, the display will limit you there too.
Do you plan to use the plugin for something else than lyrics??
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Patrick