Everyone wants this to be simple, but you really should understand that "resizing" a picture is an ambiguous phrase.
The first thing to know is that the X and Y dimensions of a picture are measured in pixels... (Picture Elements)..... basically how many little points of light horizontally and vertically.
This is what controls how "big" the picture appears... as Frenchchuffed listed, way too large for most screens, his pictures are 2048 by 1536 pixels, that is why they are SO BIG !!!
(hint, make most pictures display around 1024 x 768 if you want to fit most pages)...
OK that is what most "resize" operations do, and they normally will tell you in PIXELS what the size is..
But you can run into another problem area....
Size as in the size in Bytes of the file that is the picture..... this is related to the compression of the picture, where a mathematical function is applied the picture to make it still look good, but use less information... changing the compression will NOT change the PHYSICAL size in PIXELS.
Why do I tell you about this? Because really good cameras can make HUGE files in the file size, and may take forever to upload, display, or the site just flat refuses to display.
So you want to be aware of both. Good software programs let you control both "types" of "SIZE".... crummy ones will only do the PHYSICAL size in PIXELS.
I use IRFanView, for the last what 20 years, free, comes in 64 bit version and does both PIXEL RESIZE - called resize in the program, and FILE RESIZE - called compression in the program.
Greg