Hi Lisa,
You can increase the excerpt word to increase the character limit. But one thing if you do this then all content from the excerpt will increase. So its better to customise the code but We recommended not to change the code because if you change the code then while updating to our new version all your customisation code will be lost. So better make child theme and first unhook the functions and the edit the code.
http://codex.wordpress.org/Child_Themes
Thank you!
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