Size means two different things and people are rarely asking about the one you assume. This is about megabytes, not pixels.
Dimensions are how many pixels wide and tall the image is. File size is how many bytes it takes to store them. You can reduce one without touching the other, and which you need depends entirely on why you were asked.
An upload limit in megabytes is about file size, so lower the quality. A requirement to be 600 pixels wide is about dimensions, so resize. Doing the wrong one produces a file that still gets rejected, which is where most of the frustration comes from.
Quality reduction on a photograph is remarkably forgiving. Dropping from maximum to around 80 per cent often halves the file with no visible difference, because the detail being discarded is detail your eye was never going to notice.
Change only the quality setting and leave the dimensions untouched.
Below about 60 per cent quality most photographs start showing visible artefacts around edges.
No.