Where are you trying to set the size? Within Pixlr or within the setting for the printer's driver? It's probably the latter that should be the important one. (i.e. forget about the setting in Pixlr and concentrate upon those you'll find after clicking on 'Printer Properties' (or whatever else it is that you see) in the dialogue box after you've pressed Ctrl-P.
Alternatively, do as I do when I want an exact image size (e.g. for framing) and import your image into a blank Word document (or, for greater flexibility, into a fresh document in a DTP program) and then print that document.