Long Equations in Latex

I recently need to solve some nonlinear least square system, and the equations soon become very complex for an A4 page to display.

Of course you can solve it using eqnarray and show it in multiple lines, but it won't work if the equation cannot be separated, such as it is a long fraction.

I resolve this by adding \small before the equations, and add \normalsize after the equations. Actually, there is something else you can use:

1. \tiny
2. \scriptsize
3. \footnotesize
4. \small
5. \normalsize
6. \large
7. \Large
8. \LARGE
9. \huge
10. \Huge