Hi,
In this page:
wena.preart.kylos.pl/index.php/sklep/plaszcze
The thumbnails sizing is really depends on how many products per row you are listing in the category browse page.
If you list 10 products in the row, of course the thumbnail sizing will be smaller (both height and width respectively) in order to fit 10 products in a row. But if you set 2 or 3 products per row, then the thumbnails will be larger. (maximum 350pixel in your case both for width and height - which ever comes first)
As i am seeing, you have set "4 products per row" through the Virtuemart Configuration. Try to change that number to 3.
from here:
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If you set the number to 3, the thumbnails will be larger. (the maximum value they can get will be 350pixel, which is the original resolution of your thumbnail.)
Regards,
Support