Screen shots of your plug-in dialogue boxes would really help. I don't recall any image reduction/enlargement setting in the plug-in, so it should import your image at 100%.
Using roll paper, you still have to specify a "page" size. This must match the width of your roll paper. For example, if you have 24" wide roll installed, then your page size should be something like 24"x36".
If your print is smaller than that, you simply specify your image placement within that page. Usually it's 0.25 from the top edge (for a small white border), and centered within the width of your roll. Turn on "conserve paper", and all should print correctly.