If you looked at Nokia's diminutive Treasure Tag WS-2 and said "I like the idea, but that's too much of a thing", then the Treasure Tag Mini WS-10 might just be the Bluetooth beacon for you. It popped up recently with nary an announcement from Nokia, and it's a little 'un. Measuring in at the same 30mm x 30mm face, the Treasure Tag Mini is nearly half the as thin (5.8 mm vs. the full size's 10mm) and less than half as heavy (6.2g vs. 13g). It's a darned light and little tag, but with that save bulk you lose some features.
Most notably, the Treasure Tag Mini doesn't include NFC, so pairing will have to be done manually. The Treasure Tag Mini also doesn't include a buzzer speaker, so if it's separated from your Windows Phone only the smartphone will sound the alarm. The battery is also significantly smaller (1/3 the capacity of the full size Treasure Tag), but with the reduced feature set also comes savings in battery life — you'll still get 120 days of life out of a CR2016 coin cell alkaline battery.
Additionally, the Treasure Tag Mini loses molded plastic loop in favor of a simple and flexible band looped through a hole in a corner — and your only option color-wise is white with the orange band (though with how the band is attached, swapping it out for your own loop shouldn't be an issue).
Apart from all that, the Treasure Tag Mini WS-10 should work exactly the same as the full size Treasure Tag WS-2. We don't have a price yet for the Treasure Tag Mini, but considering that the thicker and feature-packed version is $30, we'd expect the Mini to be at least a little bit cheaper, if not a lot so.