How to ask the editor of a journal whether our manuscript fits the scope or not
Broadly, I would encourage you to review the information for authors provided on the journal's website in greater depth. First, it seems that some of the feedback you are seeking is related to the format of the manuscript (you mention asking about length, for example), and as these requirements are generally laid out online in the journal's instructions to authors, the presubmission inquiry is not an appropriate venue for getting those types of questions answered. 

Second, many journals that regularly field presubmission inquiries will have a description on their website of what this inquiry should include and where to send it, although the level of detail they provide may vary. I refer you to PLOS Biology's Presubmission Inquiries page for a good example (http://journals.plos.org/plosbiology/s/presubmission-inquiries), as the points they ask authors to include in their inquiry could, I think, be broadly applied in cases where the journal has not provided much guidance regarding what they're looking for. Bear in mind, however, that it is necessary to do your own research regarding your particular journal's guidelines; Nature, for example, wants a fully referenced first paragraph of your article. 

Final point--a positive response to a presubmission inquiry does not guarantee that you will receive a favorable review, or even get reviewed at all. It is a good start to the review process, but just a start.