I made the Vanilla Shortbread but managed only to get 18 small squares from the mixture. Does the Swiss roll tin have standard dimensions? My Swiss roll tin may be smaller as I thought the shortbread was a tad too thick, but it was still very tasty.
A Swiss roll tin (jelly roll pan) is a specific tin that was designed to bake a thin sponge cake that is then filled and rolled up. The tin is shallow, usually about 2-3cm (3/4-1 inch) deep and rectangular. The pan size is usually about 23 x 33cm (9 x 13 inches) but there are many manufacturers so the dimensions may vary by a centimetre (1/4 inch) or so.
The tins are available in many kitchenware stores and on-line. For the Vanilla Shortbread recipe (from Nigella Summer and on the Nigella website) these tin dimensions give you 33 rectangular shortbread biscuits (cookies) that are each roughly 3 x 7 1/2 cm in size.