I have for example an image that is 1.10Mo which has a size of 1200 * 1600 and that I want my images to be at most 100 KB what is the size of this image?
The size (in bytes) of an image when resized (in pixels) is not deterministic (for most file formats that you would actually use). That means it's not possible to predict the file size of a smaller image size easily.
If you're targeting the maximum pixel size for a target file size, you'll have to generate a number of samples to determine which one you want to use.
Note that there are quite a few parameters which can lower file size for the same image size, but may negatively impact perception quality.