Last year, I submitted an original research paper (approx 3000 words) to a public health journal (USA). The editors advised me that they were unable to assign it sufficient priority to allow publication as a full research paper, however, they invited me to resubmit it as a research summary article (approx 700 words) which was published 6 months ago. My question is as follows: Would it be feasible for me to resubmit the full research paper somewhere else?
Almost certainly, but you need to take a couple of precautions.
It is likely that you gave up copyright to the summary article so you need to assure that you don't infringe that copyright. The easy way for that is to let the editor of the original know that you intend to submit and get assurance that they don't object. Perhaps they will issue you a specific license. If you didn't give up copyright then this is not an issue. It is also possible that you already have a sufficient license from the publisher even if you did sign over most of the rights.
But the other issue is that your full article needs to avoid any hint of self plagiarism. I think in this case the easy way is to put in a notice, even a footnote that a summary was previously published of the same material, along with a citation. Then it is clear to a reader what is going on and that (some of) the ideas had been previously published. This note also informs the new publisher of the older work, which is necessary in any case.
The main reason for avoiding self plagiarism in this case is just to provide complete context for a reader. This is especially necessary if there are some things in the summary that aren't in the full paper. But, over citation is a safe way to assure that there is no misunderstanding of intent.