You would need to edit the catalog template files to achieve this, since the "overview" text is truncated in the storefront template files.

Locate
{$product.overview|truncate:80}

in the following catalog template files (if present in your version of the cart) in the folder "content/skins/flat/" (assuming you use the default flat skin template):
catalog_carousel.html

catalog_display.html
catalog_displaybox.html
catalog_table1.html
catalog_table2.html
catalog_text.html
catalog_thumb1.html
catalog_thumb2.html
catalog_thumb3.html

and look for the below line: {$product.overview|truncate:80}

the number at the end indicates, after how many characters the overview will be truncated, when exceeding the character limit given. If you want to display more characters just increase that number.
Keep in mind, that updates might overwrite any changes you make in the template files; therefore it would be wise to keep a backup copy of these changed files.
Keep also in mind, that you should delete the content of the "/content/compiled/" folder, otherwise your changes in the template files might not show up.