Hello @mineo,

In order to change the size of the image in the recent post section use given CSS.

For this go to Admin Panel > Appearance > Customize > Additional CSS and paste below given CSS.

.recent-posts-widget-wrap img {
    height: 165px;
    min-width: 200px;
}

You can change the image height and min-width according to your need.

Hope this helps.

Best Regards!!