Size isn't everything!
If you have a large SVG then background-size: auto auto; returns it in its intrinsic dimensions.
If you omit the second auto, auto is assumed.
So if you just set (15px) you are actually telling it (15px auto); the auto here is saying use the intrinsic ratio to calculate the height given the dimensioned width..
A comma is to separate adjacent SVGs that are being rendered together so (15px, 15px) is actually ((15px auto), (15px auto)) & if there actually is no second SVG the second pair is dropped.
Other Gary has changed his code to (0.78125vw), which is (0.78125vw, auto) or (0.78125vw, 0.78125vw). 'vw' is viewport width and a value of 100 would be "fit to the width of the browser". This strange value is actually saying repeat across the browser 128 times. If you drag the browser wider, you still get all 128 boxes, only bigger.
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