The team with the biggest capacity is the home team.

hosh13 wrote: How is this determined?

Whichever team has the largest capacity at the time the match is scheduled will be the home team.

hosh13 wrote: I have 2 away league games ahead and was planning to be building in all 4 sections of my stadium after this Wednesday's game but if I end up with a home regional in 2 weeks then this obviously changes things.

That's a chance you take. Perhaps you can build in 3 sections instead, so that you can get some income should your team be the home team. There is no way to tell before it's scheduled if you will be the home team or not, but if you have no sections open and a game gets scheduled, you will be away, even at a 1000 seat bot stadium.

hosh13 wrote: If I do as planned does the game see my stadium as zero seats and thus allocate the other team as the home side?

Yep.

or wait, play the game, then see where the next cup game is scheduled. then build if you have any away cup game next round.