Can I make passport size photo on iMac computer via iPhone?

Need photo of mum to apply for blue badge. She is in retirement home

Via iPhone? No idea what that means. You can edit the size of any photo in Preview or iPhoto.

I assume you don't have a proper camera, so you want to use the iPhone as camera. OK, do that. Then add the photo to iPhoto on the iPhone. Sync the iPhone with the iMac, and it will add the photo to iPhoto on the iMac. Open iPhoto, and edit the size.

Alternately, you can send the photo to iCloud or by email, and then open it on the iMac in Preview to edit. That completely eliminates the syncing / iPhoto steps. All Preview editing tools are under the "Tools" menu.