This is a comparison of the smallest digital recorders in the market today.
The smallest would be Instamic At 1.5 x 1 x 0.5 in (38 x 25.4 x 12.7 mm) the Instamic is pretty discreet and will reportedly attach to pretty much anything. It is provided with a variety of mounting options, including a magnet, velcro and adhesive tape. https://www.indiegogo.com/projects/instamic-the-smartest-microphone-ready-to-record-video-music#/story
Then there is the Tascam Tascam DR-10X micro audio recorder with XLR connection http://www.newsshooter.com/2015/01/22/tascam-dr-10x-micro-recorder-with-xlr-connection/
Tascam DR-10L Digital Audio Recorder with Lavalier Mic http://tascam.com/product/dr-10l/
PenRecorderPro SL200 Voice Recorder Pen, Audio Activated, Small Recording Device, LCD Display, PCM Broadcast Quality, Wired Remote (Slimline Series)
There's also the Sony ICD-UX533: records PCM 44.1, lav mic input & headphone out, 4GB internal flash, USB plug built in, and it's super slim.
I use 3 for weddings and they work great for groom & officiant mic'ing, never have to worry about wireless signal drops.
https://www.amazon.com/Sony-ICD-UX533BLK-Digital-Voice-Recorder/dp/B00BQ7WRAC
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