Hi newcarrots, welcome! There is no explicit crop tool, but you can achieve the same result via other means. A couple possibilities:
1. Copy the desired area to a separate target image of the correct size (being sure to use multi-layer copy-paste if you have more than one layer).
2. Shift the entire image contents towards the top-left corner as far as you need to (to remove the left/top borders) then do an image resize to reduce the height/width to your desired dimensions. Simultaneously shifting all layers is handled by Shift-Arrow or Shift-Numpad, and of course you can always undo anything as normal if you make a mistake during the process.
In general while REXPaint doesn't necessarily have a massive array of explicit tools, there are often methods of doing things with some creativity and a combination of various features that do exist, especially if you look down the full list of special keyboard commands in the manual.
Hope that helps!
