Making one big partition isn't the best idea because OpenBSD defaults the way it does for stability, data integrity and, a big surprise... security reasons[0].
That said, you shouldn't run out of space in the default partitions when building the system (ports are another story).
As far as I know the installer defaults to giving you around 2GB in /usr/src (which is more than enough to hold the source and build everything) and if the disk isn't big enough to do that, it won't partition it (60GB is big enough to get a separate /usr/src so 120GB is surely enough).
This assumes the instructions[1] are followed so everything is put in the right places and all the object files aren't dumped into the partition. Other than that, I don't know what could have went wrong.