It's small: 1 ½” High x 2 “ Wide.  It's lightweight: only 2.5 Ounces.  And it's very Bright!

A Long Lasting Light with 3 Ultra-Bright White LED's that Illuminates Up To 200 Feet In Front of Dog and 70 Feet to the Side, Dog Can Be Seen Coming 3 Blocks Away and 1 1/2 Blocks From Behind. Rim Keeps Light From Shining in Dog's Eyes. Durable Made of Polycarbonate Plastic and Nylon. Water Resistant.

Easy To Use: Comes With Adjustable Elastic Band Worn Below Existing Collar: Easily Slips Over the Dog's Head or Use Side Release Buckles. Keeps Light From Moving When Leash Is Pulled To One Side. Collar Attachment Keeps Hair Away From Light and Keeps Light Low Beneath Dog's Head. Replaceable AAA Batteries.