In a recent review of a paper, the reviewer suggests several solutions to a flaw, one of which was collecting more data. The other solution was to shorten the paper substantially. The editor said that the revisions needed before acceptance were between minor and major revisions. In my mind, collecting additional data is a major revision. The editor has not indicated any specific steps he would like taken. Is it acceptable to contact the editor to discuss exactly what he wants done before the paper can be published? For example, can I ask if he wants more data collected? Or if there he wants the paper shortened?
There is certainly no problem contacting an editor to enquire about a manuscript and review responses. It is difficult to assess the issue in your particular case but I do agree that adding new data can be a significant endeavour indeed. Shorting the paper may be a relevant point in any case but I do not see, in general how, shortening a paper would improve any lack of data although it may be obvious in the individual case.
So, if you feel the advise you have received is difficult to follow or is unclear (and that seems to be the primary point), then asking for a clarification is in place. You can be constructive and also suggest some plan of action for the parts you see more clearly and then focus on the ambiguous parts in your request to the editor.
The gist may be that your claim is modest and needs further substantiation/proof.
Increasing the n of samples (or including meta analysis) would increase the strength of the argument without lengthening the word count of the paper significantly.
Shortening the paper will help tighten the argument, making the claims clearer, and from the journal's perspective, align the impact of the article with length of the article.