The size of the image is determined through the css that is, if you want to show a thumbnail 40x40 you need to ship the original image such as 800x600. Because of this my page / files / weighs 6 megabytes, it is not normal, thumbnail needed.

(machine translation)

The software creates thumbnails at the maximum size specified in the ACP

And where exactly adjust the size of thumbnails?

In the image settings in the ACP.  Use the ACP live search to look for "images".

And where exactly adjust the size of thumbnails?

​Can't set max resolution of full size and thumbnail. You have 1 setting. so you either.

A. make thumbnails and user clicks on thumbnail result in non-resampled giant images.

or

B. make images larger to start with (the full size you like) and go without thumbnails

Sorry, but you can't do both!