Cast iron: one can of worms, pull up resistors another?

I have another can of worms I want to open up. I'm sure I'm going to
get many opinions on this one but, let us see.

My question today is what value resistor(s) work best for use as pull
up resistors. I would like to know what value would be the best
choice and on what that recomendation is based on.

I've purchased two of the Camtronics interface boards since October
of last year and on one Dan supplied 4.7 K resistor networks and on
the other 10K resistor networks. The first was used with his 5A
stepper drives the second was intended to be used with a Gecko 320's
setup (and yes I know the pull up resistors are in the drives already
eliminating the need for external pull-up resistors) but I am
wondering about why the huge differance in values in the pull-up
resistors supplied. What formula should be applied when choosing
these values and why?

The reason I ask is that I am breadbording my own interface for use
with EMC on a mill/drill. Anyone care to comment?

Donald Brock