http://www.deliaonline.com/recipes/victoria-sp onge-cake,2158,RC.html
This fills 2 7 inch round tins. Therefore that's an area of about 77 square inches (2 x 38.5). It also tells you how much to add for two 8 inch tins, which is a total area of about 100 square inches.
Your one square tin has an area of 81 square inches. Therefore is somewhere in between... Obviously if you want to make two to put jam in between, then that's a total of 162 square inches (assuming all cakes are the same thickness, obviously), which is about the 7 inch recipe + the 8 inch recipe. A little bit either way shouldn't hurt, you'll just have to adjust cooking times accordingly.
Hope that helps! Now I want some cake..