It may seem hard to do at first. At the moment I clicked (after selecting to sign) the image was pasted and seemed not to be resizable, because there wasn’t a visible box to resize. The idea is to ignore that and act as if dragging a larger rectangle - which I cannot see while dragging. But once created, clicking the image should make the resizable rectangle visible, until the pdf is saved.
But there may be a problem: after saving and closing the pdf —with the stamp signature made in Okular and the document simply saved, not ”printed as pdf”— the document is editable in Adobe Reader, the signature is there just a ”stamp” that can be edited, resized separately, even deleted, unlike a signature added with Adobe Reader: an Okular stamp-signature it is not a ”signature” in Adobe terms. A solution is to ”print as pdf”.