I don't know the answer, but I remember having problems with EEikEdwinUserSuppliedText too. Instead I determined the editor content's height in dialog's PostLayoutDynInitL using the following
then adjusted heights of dialog, line and control accordingly (also called ResetLineMinimumSizes()).
CTextLayout *pTextLayout = pEikRichTextEditor->iLayout
/* iLayout is protected, fix is omitted here for simplicity */;
const TRect firstParagraphRect(pTextLayout->ParagraphRectL(0));
const TInt formattedHeightInPixels = pTextLayout->FormattedHeightInPixels();
const int niceHeight = formattedHeightInPixels - firstParagraphRect.iTl.iY;