Ok, I understand but what happens if I chose to display the_content and at the beginning of the_content I have an image?
That still won't work. It needs to be a post thumbnail in order to show up at the thumbnail size.
When I use the widget fresh it shows 10 posts (full text with not-resized image) even though the content_length parameter is set to -1.
if the_content is set to -1, then the full post will show. that's expected behavior.
The resize functionality only works with post thumbnails, not any images from within the post.
I change -1 to 30 and it shows the excerpt with 30 words, but does not show the thumbnail even if I set the widget to show the thumbnails and set the size for them.
You still have to set a post thumbnail for each post you want an image for. There's a new panel called post thumbnails when you go to write/edit a post. Upload a new image, and it will work.
I set the posts_per_page to 3 but the widget keeps showing 10 posts. I change the content_length parameter to -1, and this time it only shows the titles.
Not sure why that didn't work. Try deleting that widget, and drag and fresh new one over. If that doesn't work, try downloading the plugin again. The latest version is 0.4