Hi Vanina,
You can directly edit the pixel size by clicking on the "Edit" button in Sequence properties :

By default Icy try to get them from the image metadata but depending the image format the information may miss or be incorrect.
The scale information in the plugin detector is not related to the pixel size information by the way, it works directly in number of pixel : by default the scale 2 is enabled and it allows to detect spot of 3 to 4 pixels in size, scale 3 is used to detect bigger spot (5 to 8 pixels) and so on....