Sub-Panel Feed Size

[Unregistered]

I am installing a new sub-panel to replace an existing 40 year-old panel currently in the bathroom! It will be a 100 amp panel and although it is not required, it will have a main braker on it as our main breaker is outside on the power pole (a rural acreage). What type/size cable should be used to feed this new panel from the existing box on the outside of the house that has incoming underground cables from the power pole and the feed going off to the old bathroom-located sub panel? Does the new feed have to be inside conduit - if so what type of conduit?
Many thanks
Peter in Alberta