How can I resize photo on smartphone?

Are you trying to resize before or after you have taken them?

Depending on which phone you have, you can change the dimensions in the camera settings before you take them.  If you are trying to resize after you have taken them, you might have to transfer them to another platform for resizing.  There are apps out there which can resize, also photoshop.com's mobile site is supposed to have this feature as well.  I've never personally tried any of them so I cannot say for sure.