How to ask the editor of a journal whether our manuscript fits the scope or not I am interested in submitting a manuscript to a certain scientific journal, because it is well ranked. The problem is that I am not sure if my manuscript totally meets the topics and requirements (e.g. lenght, etc.) of the journal.

I would like to send a letter to the editor of the journal in order to find out whether or not they believe my paper can be interesting to the readers and may fit the scope and conditions of the journal.

I have been told that, in cases like this, this option is better than just submitting the manuscript and getting it rejected.

So, my question is: Could you recommend me a letter model to this purpose?

In addition, in this similar question: [ When is it appropriate to contact the editor before submission to avoid rejection without review? ] they talk about even sending the whole manuscript to the editor in this pre-submission step! Actually, I was thinking of not even including the abstract of my work (but just a description) in this letter to the editor. Should I really include it?
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.