It really depends on your draw length and poundage
Go with the heaviest arrow you can build that can shoot 285 - 300 FPS. Its all subjective from there.
If your arrows are too fast, they might be too light and can be affected by wind. Heavier also penetrates better* to some degree (* because some people will get triggered by this). And a relatively heavy arrow will be more resilient to deflection by twigs or leaves if you gota shoot through some junk at your Booner Buck.
If its too heavy then a deer can easily duck or move, causing a non-lethal shot or miss, at 25+ yards. Additionally, an arrow that flies the flattest will give you the most forgiveness in terms of your up/down miss.
For the reasons above, that's why I believe the 285fps - 300fps is the sweet spot. Get the heaviest you can build.
Size: 4mm is my favorite since it penetrates better and doesn't get affected by wind as much.