Researching online for a blog post on the "Masakado Mound" in the Otemachi business district honoring Lord Taira no Masakado, I landed on the Marunouchi Police Station website. Each Tokyo Metropolitan Police station has its own site; Marunouchi's feature is "ONE Spot famous sites and historic places" for each police box. The "Wadakura-mon Gate Foreign Affairs Police Box" page = photo above = about 10 minutes' walk from Palaceside Building in Takebashi includes Masakado Mound with photos (http://www.keishicho.metro.tokyo.jp/1/marunouchi/koban/wada.htm). Its tagline: "A police box with a slightly long name, home to nesting spot-billed ducks and Masakado Mound." Hmm? Seven kanji—is that so long? "My nearest box has eight!" I muttered to myself.
Spurred by curiosity, I skimmed Metropolitan Police jurisdictions: the longest names seem to be "Odaiba Kaihin-koen-eki-mae Police Box" (Tokyo Bay Station) and "Minuma-dai Shinsui-koen-eki-mae Police Box" (Takenotsuka Station)—11 kanji each (sorry if I missed others). The latter reads "minumadaishinsuikoenekimaekoban"—21 hiragana, beating "Odaiba..." by one character for top spot.
Browsing station pages, some usability issues appear. Hachioji and Tsukishima stations lack a "Police box" menu item on the top page—you reach maps via "Jurisdiction map" or "Community activities," then click boxes on the map (sigh). Tama-Chuo and Mita show boxes on maps but no individual descriptions or photos. With dedicated websites, ease of use deserves attention too.