Unsurprisingly, your research statement addresses at least two very-different audiences, and in your specific situation, maybe three or more. The usual entirely-true cliche is that most people on the postdoc-hiring committee will not themselves read beyond the first page or so of the whole thing, so you should be sure to make your big points, of course necessarily in rough terms, just on that first page. In usual scenarios, there is also the cadre of "specialists" whose strong support is necessary, if not sufficient, to get the postdoc offer, and the rest of the thing is written for them.
For people in a "new" field, or in a novel interaction of two existing "specialties", I'd think you'd want to as-quickly-as-possible make the point that there is at least some interest for specialists in both those specialties in your application/combination of them... or you may find yourself with "no constituency at all", rather than the union of the two.
There are two sorts of "persuasion" that successful research descriptions typically have. Often enough, proving one's adherence to some orthodox school of thought/research, in effect sanctioned by famous people at the best institutions, is a useful/necessary assertion of pedigree. At the same time, in some ways opposite to the spirit of that, is making a persuasive argument that one will do interesting things with the materials described, so that it will be stimulating (not merely "prestigious") to have you around. The conflict with certifying your orthodoxy is that progress and constructive innovation typically involve not doing things as always done before, no matter by whom.
So whatever the length of your research description is probably fine, if you just prioritize things so that the first page is an adequate representation of your ideas for non-specialists, and the rest is persuasive, rather than merely "descriptive"... the latter lending itself to boredom and not-being-read.
