For everyone willing to resize thumbnails, while keeping the picture aspect ratio and showing a center-cropped picture (no matter if portrait or landscape)
I've finally found the relevant CSS code.
Well... almost! 
The following code sets all thread thumbnails (
where there's an attachment for it) to be shown on a round bordered rectangular of 80px by 60px, while leaving all other thread's pics with the same default avatar size. I've set my avatars to be round, so I see a mix of bigger rectangular pictures and small rounded avatar, and for the moment I'm fine like this.
I've also left blank the default thumbnail field in AL Thread Thumbnails options, because I want to show members avatar if there is no thumbnail to be shown.
But if you set a default thumbnail image, it won't get the set thumbnail's size as it will stay the same size of user avatars.
Here we are, put this in your
extra.less:
Code:
.threadThumbnailWrapper div.avatar
{
width: 80px; // CHANGE this value for a different width (also change max-height below to the same value)
height: 60px; // CHANGE this value for a different height (also change max-height below to the same value)
border-radius: 5px!IMPORTANT; // this is making rounded thumbnail corners: INCREASE or REDUCE if you want them more round or less round, comment or delete if you want square corners
}
.threadThumbnailWrapper div.avatar img.alignThumbnail
{
object-fit: cover;
min-width: 100%; // this will scale width to full thumbnail witdth for centered cropping - DO NOT TOUCH
min-height: 100%; // this will scale width to full thumbnail height for centered cropping - DO NOT TOUCH
max-width: 80px; // CHANGE this value for a different width
max-height: 60px; // CHANGE this value for a different height
}
// the following CSS will scale and center crop the smaller thumbnails that are showing on widget in default avatar size
.threadThumbnailWrapper a.avatar img.alignThumbnail
{
object-fit: cover;
min-width: 100%;
min-height: 100%;
}
All other code combinations that I tested, didn't came up as expected. I turned to get stretched and deformed user avatars on "What's new" and all places other than "forum_view" and a few other issues that I don't even remember.
I can't go further than this as I'm not a coder, so if anyone else finds a fix for the missing code,
please share.
@AddonsLab please 
implement this as an option in next release, and please also add those options to choose if you want to show same size default thumbnail or not and possibly some more places where to show this or not (like exclude widgets) and so on.
Also, is it possible to turn the forum IDs fields into a forum names selectable list, so it would be easier to set up?
Thanks for your time
