I am trying to cropa picture but it has to be 1546 X 423 it has to be this size. I don't know how to do it
You can't really crop a photo to an exact size, you can only crop to the proper aspect ratio and then set the size at export.
So, set the crop tool to crop to aspect ratio 1546x423, do the crop, then in the export dialog box set the size of the exported image to 1546 x 423.
I suspect that you can't get the exact aspect ratio that you need, and thus your export will be off by a few pixels, but I haven't tried it.
I just verified that dj_paige's instructions are exactly right. (I didn't doubt his instructructions but did wonder if this would trigger an existing bug in Crop -- it doesn't.)
Except for that bug, this method will always generate a pic of the desired exact size (as long as each dimension is no longer than 10,000 pixels -- if they are longer than that, then you need to divide each dimension by 10 when entering the custom aspect ratio).