For long equations, you can use specific environments for multiline formula. You can find a lot of informations on the new Math.lyx manual : http://www.lyx.org/trac/browser/lyx-devel/branches/BRANCH_1_5_X/lib/doc/Math.lyx or in PDF http://wiki.lyx.org/uploads/LyX/LyXDevelDocumentation/1.5/Math.pdf
Siegfried.
