but then it is displayed so that it is too large to fit in the field.
You are not trying to write proof of Fermat Last Lheorem, are you ?:-)
Assuming you meant that the equation is overflowing to fields, and also assuming that you want to avoid this, instead of changing the font, I would suggest amsmath . For example, you can write a long equation as follows:
\begin{multline} A = \lim _{n\rightarrow \infty }\Delta x\left( a^{2}+\left( a^{2}+2a\Delta x +\left( \Delta x\right) ^{2}\right)\right.\\ +\left( a^{2}+2\cdot 2a\Delta x+2^{2}\left( \Delta x\right) ^{2}\right)\\ +\left( a^{2}+2\cdot 3a\Delta x+3^{2}\left( \Delta x\right) ^{2}\right)\\ + \ldots\\ \left.+\left( a^{2}+2\cdot (n-1)a\Delta x +(n-1)^{2}\left( \Delta x\right) ^{2}\right) \right)\\ = \frac{1}{3}\left( b^{3}-a^{3}\right) \end{multline}
(An example is taken from Mathmode : necessary if you want to type good math.) There are many other environments in amsmath: align and its variations for line alignment, gather for combining several equations, multline (note, no i ) for a multi-line equation, split etc.
To answer the second question: you can get an unnumbered equation using the star form of the equation environment:
\begin{equation*} E = mc^2 \end{equation*}