Considering a decent packer is 36-38 inches at the shoulder and then you allow some extra for their head (without horns) You are looking at a minimum of about 40 inches tall. If the goats get along well they can ride in the same box loose. If they butt and fight then you'll probably want to tie them one to a side. I'd make it as large as you can. Normally 4ft wide will slide between the wheelwells of a pickup unless you have a small one. Then make it as long as you can. Making it fit snug between the wheelwells will help keep it from sliding around in the back of the truck. Holes large enough for them to stick their heads out would be a bonus. Avoid holes in the front that would let windblown rain inside and don't use bedding that will blow around and get in their eyes.