I use an iPhone . When I press on someone's profile pic, a list pops up. I can choose "add to photos ". Then I can see it bigger Shen I view my photo app. I don't know if you use iPhone for Ask though so I answered best I could .
If you're using a PC or a laptop you can simply hold down the control key while you rotate the mouse wheel. Rolling it away from you makes it bigger, toward you makes it smaller. It enlarges the whole page of course but the photograph enlarges with it.