The search result is going to show the title of the page or article so the size of that depends on the page or article title.

If you are using a user.config file, then we have default settings already in user.config.sample for search results highlighting which can show a fragment that matched the search, you can control the size of the fragment with this setting:

<add key="SearchResultsFragmentSize" value="300"/> 

We have different defaults in Web.config for backward comaptibility.

If you were not previously using a user.config and therefore not using results highlighting you will need to rebuild the search index after enabling it.

These are the settings that should be in user.config

<add key="DisableSearchFeatureFilters" value="false" />
  <add key="SearchUseBackwardCompatibilityMode" value="false" />
  <add key="EnableSearchResultsHighlighting" value="true" />
  <add key="SearchIncludeModuleRoleFilters" value="true" />

if you override the fragment size you should also put that setting in user.config

See also

Site Search Overview

Rebuilding the Search Index

Hope it helps,

Joe