A slice of beef should be rather long, about 20-25 cm/ 8-10 inches, about 10-11 cm /4-4.5 inches wide, weighing between 160 g – 200 g/ 5.6 – 7 oz each.
You can use thin round steaks. Flatten them with a meat mallet, and if they are not long enough, layer two of them to get the length you need.