I guess you are talking about the aspect ratio, relationship between the horizontal and vertical, rather than file size in megabytes or pixels.
I think iPhones are 4:3, but the aspect ratio can easily be changed in pretty much any editing software. Widescreen is 16:9, and other common formats are 3:2 and square. Micro four thirds are typically 4:3 and “regular” (aps-c) digital SLRs are normally 3:2 as are full frame SLRs.
It really only matters if you want to display your photos in a certain way to emphasise the image. For example I recently exhibited a shot of a tree on a plain in 16:9 to bring out the wide landscape and sky against the lonely looking tree. Also you might have something like a digital photo frame or monitor with a fixed aspect ratio and you want to fill the frame.
A very easy app to use is Snapseed with which you can change aspect ratio etc very easily. Not sure though if it is still available on iPhone, I think maybe not. However Photos has an editing facility.
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