Lilacs grown as hedges are pruned often, pruning won't hurt it. Remove anything detracting from the lovely shape you see in your mind's eye, however tall that ends up being when you're finished. It will grow again and should bloom profusely next spring if the pruning is done soon.
A never-pruned lilac can form a lovely, small tree. Once pruned, it can never look like a tree again. If you start with a new one, you might enjoy it more as a tree, which never needs to be pruned (except, as in the pic, if put so close to something that it needs to be stopped from bumping into that something occasionally.) I didn't plant this tree, but leaving it was the saddest part of moving away from this particular house.
