I need to vastly reduce the size of a Pyracantha that is about 10' high and 4-5' diameter. When is the best time to do this and still have berries in the winter?
Pyracantha can be a brute if its left to itself. You could prune selectively removing branches with no fruit, or wait till spring and cut it back hard.
Ally, they flower on 2 year old wood so you need to retain some of that to get flowers which obviously later become berries. As Dave says, it does respond well to hard pruning so you could cut back hard a third of the stems for the next 3 years which will mean 2/3rds of the shrub will flower next year and one third the year after. By then the new growth from the first pruning will be ready to flower. If you keep doing that, 2/3rds of the shrub will always be in flower and you keep the size under effective control.
If you wait until after it has flowered in late Spring, you'll be better able to judge what to remove and what to leave.