Getting the correct size of the buttonhole is a test and trial sort of thing. You set the width and length aproxymately from the marks on the attachment and then fine tune when test sewing (like any buttonholer function really). I have the black one, which machanically is identical to your white and red. The only thing that took me while to understand is the importance of using a relatively large size needle and to match needle and thread size suitably.
I have no problem using Gütermann's sew-all polyester and but some cotton threads have turned out the snap easily, most have been fine though, and part of my troubles have been tension issues. I think Singer recommended mercerised cotton of best quality. I don't know what type of machine you plan to use it on. My 201K can have the feed dogs lowered and it works well, some machines need the feed dog cover. It's easy, just do a bit of test sewing and try out the adjustment options and you will get it.