i'm currently doing home improvements and making the house dog friendly and have a really stupid question but i hope someone will answer it anyway - i'm getting a new security door as the last one is flimsy and unsafe i want to put in a dog door at the same time the sizes the company offers is 240x190 (Small) or 400x260 (large) will a sheltie be able to fit through the small dog door or will i need to get the large fitted?