Q.pruning tuberous begonias
I potted my tuberous begonias too soon and they have grown perhaps too well. We have at least another month of frosty weather; should I cut back growth to (say) six inches. I have two plants from the Bolivean type as well in the same condition. Fibrous types are being grown from seed.

These links will help you.
https://www.gardeningknowhow.com/ornamental/flowers/begonia/pruning-begonias.htm
https://www.gardeningknowhow.com/ornamental/flowers/begonia/wintering-begonias-overwintering-a-begonia-in-cold-climates.htm