I often use about 4 liter water to spawn (30x30cm about 5-7cm deep). So IMO tank size doesn't matter. They will spawn in a cup of water..... but you'll risk killing the female. BUT you will have to move them to a bigger tank/tub after 2 weeks. And in that 2 weeks you need to do more water changes. ...Almost forgot, if glass tanks are hard to get, you could use plastic tubs. Just make sure they are safe for humans and your bettas will be fine.
You can keep the tank in the dark or light it up - bettas will be able to see. I don't use extra light, I use a normal day-night schedule. I often cover everything during windy weather - very dark. Nor do I use any half cups for nests. I simply cover the top where I want them to nest..... it's easier to control (I mostly use dark plastic tubs)
You have to culture fry food before you spawn - so they'll be ready when needed. If you don't have any cultures, you could try hard boiled egg yolk. Just feed a tiny bit and spread it evenly through out the tank - 2 or 3 times daily. Make sure to change 50% of the water daily.
Infusoria is a good starter but it must be ready before the fry are free swimming. Perhaps you could add some creek/river water to speed up the process.
You CAN NOT feed newly hatched fry frozen food. Not even frozen daphnia because they are too big for the fry. Betta fry are usually picky eaters, unlike other fish.
SO how are the pair acting .... are they both still vibrant? Or have you decided to postpone and separated them.