What species of willow is it? Pruning no later than now is probably okay. You can usually be very hard on most species as they grow very quickly, and seem to recover from a hard cut back with equal reaction. Worthy

Weeping Willows will require a fair amount of creativity in pruning to reduce their size, without ruining their shape. Probably best to take it in easy stages, over several years, always cutting to side branches--no stubs!