I have a lilac bush that has gotten out of hand. It's about 6 ft tall now,
and not very thick. How far back can I cut it? Will it grow lower and
thicker if I cut it back?

I have what I think is boxwood with thorns, also. They are also too tall,
about 5'. I'd like to cut them back to about 3'. Will they survive such a
drastic pruning?