Reduce the image field

I would like to reduce the size of the image on which I work, to work only on the interested area and thus move from an image of several thousand square kilometers to a dozen hectares. How can I do this without reducing the quality of the image?

It depends, if you’re processing SAR data, you should multilook. If it’s optical data then resample. In all cases you’ll be reducing some quality from the data.

I use the maps of Sentinel-2, with 12 bands, so I just need to resample if I want to reduce the image field?

For Sentinel-2 you should resample to the resolution you would like to use. For example 20m.
And afterwards you can use the Subset operation to cut out the area you are interested in.