I picked this up on a whim at CHICAGOPEX this past weekend. Definitely diminuitive at 75mm x 43mm (2.95" x 1.69").
Doing some research online for comps, the examples I came across had eye-watering prices:
1. Offered by an eBay seller in Australia at US$300 shipped, with the descriptor "Printed to Private Order Advertising Postcard for Austin Walsh Tobacco Company. This is the worlds smallest piece of postal stationery."
2. Shields Stamps in Australia at AU$425 (US$287)
3. Three examples at classicstamps.co.nz ranging from NZ$139 to NZ$217 (US$87 to US$136)
4. 3 Centuries Postal History in the UK at 85UKP (US$103)
So apparently more scarce than I realized (or the dealer who priced it at US$10).
Other references also make that claim regarding smallest advertizing postal card:https://nzstampcards.co.uk/austin-walsh/