30 litres.
and yes, a biorb is perfectly fine for a betta, just stick a gang valve after the airpump to adjust the flow.
you can buy a heater pack for it, which allows you to put it in tropical temperatures.
the biorb filtration i might add, is highly efficient, my one is probably only marginally less effective than an equal amount of efisubstrat.
and biorbs are brilliant! i've bred killis, shrimp, and kept loads of fish in great conditions in there.
i got mine for £100 when they first came out, they are quite expensive.
as for water changes, there is carbon and zeolite in the filter pack (if you unscrew the bubble tube theres a small sponge and under that the absorbative stuff) in there, so it does help keep everything up and going.
just make sure you plant it well, they can be quite susceptable to algae due to the lensing effects of the acrylic