It's hard to point at a specific piece of documentation to solve our problem. Here's the code you're looking for:

CloudDeploy[
 FormFunction[{"size" -> "Number", "image" -> "Image"}, 
  ImageResize[#image, #size] &, "GIF"]]

You can change the size of an image using the ImageResize function. To find that, you shave to search the documentation and learn a bit about how image processing works in Mathematica.

"Number" and "Image" are "Interpreter types". See the documentation of Interpreter for more examples. These "Interpreter Types" can be used as inputs to a form function. There are many of them.