People say "image size" for two different things, and mixing them up causes a lot of confusion.
Dimensions (pixels)
The width × height, like 1920×1080. This decides how big the image appears.
File size (bytes)
How much storage the file takes, like 450 KB. This decides how fast it downloads.
They're linked but not the same
A 4000×3000 photo can be 8 MB or 800 KB depending on compression — same dimensions, very different file size. And two files of the same KB can have wildly different pixel dimensions.
How to control each
- Too many pixels? Resize to the dimensions you actually display.
- Too many kilobytes? Compress to shrink the file. To hit an exact target, see compress to 100 KB.
Often the best result comes from doing both. Related: reduce size without losing quality.