Hello,
I'm using k2 on a site, to represent a product portfolio:

www.nuzoo.it/it/servizi-ingegneria-prototipazione/portfolio-prodotti-prototipi

The strange thing is that when I load the images of the individual articles, they do not respect the size. I'll explain.
If you take the article "Spruzzatore di essenza" you will see that the image that represents it is the XSmall version of the original:

www.nuzoo.it/media/k2/items/cache/f7abac252ae5ed68121b92ba7a669d87_XS.jpg

but its Small version is smaller!

www.nuzoo.it/media/k2/items/cache/f7abac252ae5ed68121b92ba7a669d87_S.jpg

How come this behavior? Ideas?

Hello,

You need to double check K2's settings and the image sizes to see if there is an issue in the setup.
Keep in mind that in the category's settings you can override these sizes, so you need to double check these settings as well.

Yes, I know that category's settings can override these sizes. Indeed I have a category that put all the images in xsmall version. But some images have xsmall version bigger than small version, this is strange.

How can I set a dimension only for an article, without inheriting from a category?

You might have confused the category's and image settings.
K2 image settings specify how large a specified image size is. In the category you can override these settings (not an ideal solution) and you can choose which image size is being used in each view. The latter is the setting you need to choose.

Is the XSmall is larger than any other size, then either the Image settings (check K2's options) are wrong or at some time the settings were different and older images reflect these changes.