This poster with the venue was posted on IME Entertainment Group’s Facebook page (and is also now on BABYMETAL’s website tour page). No ticket information or info on the other shows from the tour so far.
The venue is the NTU (National Taiwan University) Sports Center. The poster states they will use the first floor, which is (via their wiki) a multi-functional court. I couldn’t find the capacity, but I did find a few pictures:
Location – Website
The venue’s main hall has 5,500 capacity, but it seems BABYMETAL will not be using it.