Trouble with keeping it responsive

I’m using the grid on a lot of archive pages and the blog page, but when I narrow down the browserwindow, the .one-half-teaser blocks don’t get smaller. they immediately drop underneath. I want to keep them getting smaller until let’s say max 250px, but I just don’t succeed in doing so.

I’m using Genesis with Dynamic Website Builder and a custom theme.

My website can be found here: http://www.jacktummers.nl

https://ww.wp.xz.cn/plugins/genesis-grid-loop/