There are dwarf varieties, actually. Many local nurseries have them on display along with the normally sized ones, in season. They tend to produce narrower, less vertical leaves but are definitely phormiums--and maybe this more relaxed look would happen to be better for what you are trying to do anyway.

Look for Platt's Black or Amazing Red - PB is a very dark bronze and AR is well, red :-) Both should not exceed 3 feet in height. Width tends to be a factor of time - the longer you have them, the wider they grow, but they can be divided. Phormiums are becoming much more common in this area as well as less expensive than in the past - you shouldn't have too much trouble locating either a bit later in the season.

Thanks, everyone, for the help. I had read that the dwarf varieties were so new that true height hadn't been established yet and that they would probably be quite large. Sounds like I either got bad info, or old info. Anyway, Amazing Red sounds perfect.

Dwarf forms have been around long enough to determine mature height - just maybe not as common in this area as in say, southern California :-) There are actually a good many dwarf forms, as not every garden can accomodate a full sized behemoth like atropurpureum or Maori Chief so hybridizers have been busy over the years. Some are quite dwarf, rarely exceeding 18 inches, but to my eye don't quite come across with the same effect as a larger form.

Here is a link that might be useful: some nice dwarf phormiums