Answer: According to Michael Dirr's "Manual of Woody Landscape Plants," there is a small textured Pekin Lilac or Syringa pekinensis that has 2 to 4 inch leaves 1 to 2 inches long and white blooms; he describes it as being "bushy, spreading and rounded" in habit although it can actually reach 15 feet or more and can also be grown as a tree form.
He also lists Persian Lilac or Syringa x persica in the form "Alba" as having 1 to 2 1/2 inch by 1/3 to 1/2 inch wide leaves on a smaller shrub in the ranges of 4 to 8 feet tall and 5 to 10 feet wide.
There are a number of small leafed and smaller growing lilacs gaining in popularity and somewhat confused in the trade, being sold variously under the names of Syringa meyeri, S. microphylla, and S. patula so it is possible you have seen a white flowered form of one of these as well. Although the Manual does not specifically mention these being available in white, S. meyeri "Palibin" has dark buds that open to have an almost white coloring. (This is actually available from Monrovia.)
I am not sure which lilac you have seen, there are hundreds if not thousands of them and so I'm sorry I can't be more specifically helpful; you might try contacting a lilac specialist such as the International Lilac Society
(they have regional chapters in the US) to see if they might be able to help you identify the plant with certainty, perhaps by using a scan or photo taken of the bloom and foliage and a photo of the plant as a whole. Best of luck in your search!
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