Unfortunately, the tightpage option of the preview LaTeX package crops the page only vertically but doesn't crop horizontally. Even the
-T tight option of dvipng doesn't help. dvipng prints the following warning: preview-latex tightpage option detected, will use its bounding box.
If you want to tightly crop the page horizontally you should use the standalone LaTeX package instead of the preview package:
\[ x = 3 + y^2 \]
The standalone package removes the left and the right space around LaTeX formula:
If the LaTeX equation is too long and doesn't fit the page you can increase the width of the page:
But it is only applicable to PDF documents. The width of PNG images is always unlimited.