When you apply a template to images, Photoroom creates a design for each image. The designs are the same size as the template by default. To change the size, you can either:
Edit the template and resize it (see Edit an existing template).
Set the designs to use a different size to the template. You can do this by using the Resize settings when you apply the template to images. For instructions, see Apply a template to multiple images in Batch Mode and Create a batch of images and apply a template.