For a mobile target, I would go with closed cell, self healing foam. It is what the new style of 3D targets are made from and they last a long time if properly cared for. You can make the vitals area out of a replaceable module of laminated cardboard or similar, so that you can replace when it's shot to pieces. Metal stakes and bands of steel are tough on arrows, so an alternative may be in order, depending on how you design the target. I bought my 3Ds. Went down to the local docks and bought some of the foam they used in the floating docks. I cut them into 20cm x 8cm discs and threw them about in the field. Even got the wife to toss a few for me and I shot them as aerial/flight shots. For a fixed target, I made up a covered target stand and used 3 hay bails compressed onto a pallet with bands that didn't cross the visible area of the backstop. I placed a life sized cardboard cutout of a deer and had drawn in the vitals. Except for the shoulder, bones aren't really a problem when shooting a deer with a 50# bow. I put a red dot on the spot I wanted to concentrate on.
As far as size goes, I would make it as close to life size as possible, as well as shape. At least as far as where the shoulder, legs and vitals are. If your roe is anything like our deer, it has a vital section that is about 20cm in diameter. From my experience, if you want to keep your shooting realistic, the heart is not a target that a bowman should look at. At least, not as a primary target. A heart hit deer can run an amazing distance before dying on the run. Hit the lungs and they will normally run a shorter distance before dying. Hit the liver or ponch and they will run until they feel safe and then will bed down and die if left undisturbed. Not always leaving an adequate blood trail to follow.
A mature boar is a whole different matter. A mature boar has a shield of gristle from the shoulder to the back of the ribcage that is nearly impenetrable with a 50# bow. The actual spot to sneak the arrow into the animal is behind the ribcage as the animal is angled sharply away. Meaning that the shot must penetrate the ponch/belly, in order to hit the lungs and heart. This makes your actual target spot to be about 8cm in diameter. A younger pig would have a kill zone of about 16cm or so, from the side.