How long do baby rabbits need milk, and how to tell their sex?

It can be quite difficult to an untrained eye with a rabbit that young, but is possible. It's best to have one person hold the bunny upside down and hold the back legs, so it can't kick. Once the bunny is calm, look for the penis (testicles will not be present) or vagina. Do a google image search of "how to sex a rabbit" to find good examples of the difference in anatomy. Below is a good link that explains how to look for the anatomy: http://www.wikihow.com/Determine-the-Sex-of-a-Rabbit

You can check their bellies. At that age, you can see a white band which is their tummies full of milk. Rabbits will only nurse once or twice daily unlike cats and dogs who nurse multiple times daily.

In a young rabbit it may be difficult but with careful handling and inspection, you can determine sex. Follow this link for directions on how to proceed: http://www.peteducation.com/article.cfm?c=18+1803&aid=2694

You will need to add some heavy whipping cream to that milk for it to be suitable for rabbits. I suggest you refer to this article on how to care for baby rabbits. ( https://www.mybunny.org/info/caring-for-newborn-baby-rabbits/ )

If this is a wild rabbit it needs to be returned to where you found it or call a wildlife rehabilitator. Wild rabbits typically die in captivity. http://www.2ndchance.info/bunnies.htm If a domestic rabbit here is more info on raising. http://www.bio.miami.edu/hare/orphan.html Much depends on how old the rabbit is. Babies are only nursed twice a day and are usually weaned by 4 weeks old.

Kitten milk replacer (KMR) or goat milk should be fed to nursing kits (baby rabbits​). Feed twice daily. Be aware that bunnies can be very difficult to hand raise. Follow the link for more information. http://rabbit.org/faq-orphaned-baby

I think the bunny looks fine. Just because it is smaller does not mean it is not healthy. You can add goat milk (regular, not low fat) to the kitten milk replacer and a pinch of acidophilus (aka Probiotic) to the formula to promote healthy gut flora. Formulas vary depending on region. Avoid Esbilac and any puppy formulas. FEED TWICE A DAY ONLY and let them have free choice alfalfa hay. At 3-6 weeks of age they should be eating about 13-15 cc/ml each feeding (two feedings a day) but it may be LESS depending on size of rabbit so they little one may only eat 8-12cc. That is OK. Hope this helps.