seems likely that your underlying issue is that your actual "gross" multi-monitor resolution is not really any greater than what you would realize out of a single monitor. the net effect of this is to take the static "root window" and stretch it accross all your extra screen real-estate, without really making anything on-screen "smaller" in relative/resolution terms.
ideally you would be able to indicate to the WoW client that your hardware-supported resolution is actually huge (1600x6000 or whatever), and allow WoW to make use of this and your graphics hardware to render it and spread it accross the multiple monitors.
assuming this is not possible... you need to shrink individual UI elements.
there is the global Blizzard "UI Scale" interface option.
beyond that,
Aloft would allow you to reduce the gross "resolution" size of nameplates, by adjusting layout, shrinking health bar sizes, using tiny(er) fonts for text, and etc.
at the same time, there are risks: i don't know if this will yield nameplates that are actually legible (the bitmaps for tiny fonts will not increase in resolution, so your text could be quite ugly), and contorting nameplates beyond a certain shape/size/placement will render them insensitive to the mouse (still visible, just not useful for click-targeting).
edit: as well, as far as i know, no nameplate addon will have any control over how/where Blizzard displays nameplates. WoW 3.3 has an "allow nameplates to overlap" option coming (see this
thread; currently available for test in the 3.3 PTR), which will prevent nameplates from jumping around in crowded sitiuations and/or from appearing at large distances from the associated character model. currently, shrinking the size of nameplates using Aloft will alleviate "packing" (or tiling, or crowding) behavior to some extent, and Aloft also allows some degree of control over nameplate "footprint" (as Blizzard implements it; in Aloft these are the "packing" height/width, available under Aloft's "Frame" options), which can also permit some control over nameplate "packing" (again, at the risk of rendering nameplates insensitive to the mouse).
in fact most addons, for every conceivable purpose, will allow some sort of layout/scale control (icon/bar height/width, font size, placement of visible elements, etc). crafting a comprehensive custom UI (or just using one someone else has built and packaged as a "compilation") would let you shrink and place things pretty much arbitrarily (with the same risks: tiny fonts may not look very good, etc).
anyway, hope that helps.