how to change size of pics on product page?

I wish to change the size of pic on the product page
but don't change the size of this pic in the category....

You should go in System-->configuration-->Files.
The thumbnails size is used on the product page and the listings (on the listings only if you didn't specify an image size in the menu/module's options) for the thumbnails. So you can increase these options there and set the thumbnails size of your listings by editing the options of your menus/modules via the menu Display->Content modules/menus.