Adafruit has the "Feather" series, which includes several BTLE-enabled boards Adafruit Feather 32u4 Bluefruit LE : ID 2829 : $29.95 : Adafruit Industries, Unique & fun DIY electronics and kits
"Size of a quarter" is probably pretty tough to achieve without building your own hardware; the "wearable" Arduino-like devices I've seen have all be semi-enlarged to allow easy connection to sensors, switches, etc...