Hi,
There is an option for that, in Config > Main > Images, you can set a sizes for image with and image height.
Image width and Image height : You can specify a maximum width and a maximum height for images attached to products. If the images appears to be bigger, they will be resized automatically during the upload process. If you do not want to resize images, leave these fields blank. These options only apply to the "zoomed" image of products on the product page and to the current category image on category/product listings. Note that if you change these values, the images you already uploaded in your products/categories won't be resized as this process is done at the moment you upload them.