Substrate isn't always needed, however, if you don't use substrate-I would use dark paper or something under- on the bottom of the tank to help make the Betta feel more secure and limit reflection.
Commercial Sponge filters have on the box what size tank they will filter.
For homemade DIY I look up sponge filters on the internet, find the size for the tank I am using, then cut my sponge to as close the same size as possible.
I'd go with the mini. The one I have has a shorter lower base. The sponge almost sits on the bottom. This gives a longer lift tube for more flow. You can adjust the flow with a $3 valve in the airline.