They're being announced early tomorrow morning, just as the markets open, and they're expected to be sizable. Kara Swisher at All Things Digital is talking about 2,000 employees, and there could be more in the months to come. The cuts will involve all units of the company - presumably including the company's sizable L.A. office - but keep in mind that Yahoo has a workforce of over. From ATD:
The layoffs tomorrow are not the end of the road in cutting costs. Along with the likely shedding of its ad tech and search businesses, Yahoo leadership is also looking at future cuts, as it evaluates current businesses, which could lop even more employees off its roster. That said, Yahoo will also be "doubling down" in some older and new arenas, so there would also be simultaneous hiring in the months ahead.