MH Posts large [lite] Image size

Hi there, i use a child theme for mh magazine lite and i made some changes there. Normaly i use MH Posts large [lite] for my static frontpage but after some changes (i made the website larger (width) it didn’t look good anymore, because the MH Posts large [lite] show me the pictures in 100% but take still the smaller original sizes (678×381).
In my case: My width-size for pictures on the website is 868. The plugin still take the 678×381 and zoom them to the 868. It looks very blurred and not very good.

I didn’t find the solution for this (tested some files but didn’t work) where i can set the plugin to 868 original with a child theme. Any hints? 🙂