an't say I'm set up to compile plugins myself - but just as a quick pointer to anybody who can...
The problem isn't that the user can't enter a longer text per se (the plugin takes a max character parameter that can be set very high), it just limited by the edit control's size itself.
The reason for this is that it is missing the ES_AUTOHSCROLL style >_<
That said... there are alternative plugins as well Afrow_UK's PassDialog, for example, or you can just use nsDialogs rather than a pop-up dialog, etc.
Are you a developer who uses NSIS to distribute your application? Are you a Winamp plug-in developer who wants to use NSIS to distribute your plug-in? Have suggestions for other people like you? This is the place.