What's the Mac shortcut for increasing the size of the online image?
Hold down the Command key and tap the +/= key to increase. Use the -/_ key to decrease.
Not sure if I fully understand your question, but a good way of making the window full size is to click on the lower right hand side of your window and dragging it down and towards your right (diagonally).
Safari offers only two choices, zoom page and zoom text only.
To zoom images, you have to look for a safari extension.
I have not tested any, so I have no information about any of it.
Retrieving data ...