IEEE journals: how do I find articles published as letters? In JMEMS there are primarily 3 types of publications: conference proceedings, letters(3 page limit) and articles.

I am trying to find articles published in letter format in JMEMS. I have access to Webofscience and ofcourse google scholar/scopus etc. I can find these articles by manually going to each issue (published quarterly) and looking for 3 page articles. However this is very inefficient, since I am looking for a certain topic published in the letter format, and I will have to go through all the issues to collect these articles and then look through them.

How do I find/search for letters published in JMEMS? Is there a more efficient way?
First things first: 

Unless you are searching for IEEE Letter papers in venues which only publish Letters (such as IEEE Communications Letters, IEEE Signal Processing Letters, IEEE Electron Device Letters, etc.), there is, unfortunately, no formal, efficient way to search exclusively for Letter papers in IEEE venues which publish multiple types of papers.

As you stated, your target venue (IEEE JMEMS) publishes multiple paper types, so it seems that some creativity is needed to generate some results which are relevant to you.

One way to get creative here: It is quite common for authors of IEEE Letter papers to refer to the paper as such within the text of the article, e.g., as "This letter ..." or "In this letter ..."

In this case you can use Google Scholar to search for keywords "letter", "this letter", or "in this letter" and restrict your search to your preferred IEEE venue (in your case, IEEE Journal of Microelectromechanical Systems). You can further narrow down the results list in Google Scholar by also providing technical keywords relevant to the papers you are interested in.

While the above is not a silver bullet, I know of no other better way to accomplish what you aim to do.