Move the READ MORE button in Post excerpt

Hi guys.
On my homepage http://happyfrogtravels.com there are six last posts displayed in rectangle fields. The READ MORE comes after 33rd word and sometimes deforms the rectangle: just observe the difference between the GUIDE Amsterdam and STORY Luang Prabang fields.
So, I would like to set it to appear let say after 20 words. How do I do that? I found on some forums that I should put a code to functions.php file. But there is no such file in the Editor.
Please give me a hand.
Thanks,
Milos