From a few of the datasheets, parameter D070...

PortB weak pull up current
50uA minimum - 400uA maximum @ 5v, pin @ Vss. A few of the datasheets say 250uA typical

This would suggest anywhere from 12.5K to 100K of resistance per PortB pin, with a 20K typical. A bit of a wide range. Fine for a switch matrix, I'm thinking a bit sloppy for anything else...