I am going to build up a cross bike from parts I have on a mountain
bike. The mountain bike frame is way too heavy and rather small for
me to ride comfortably for any length of time.

My question is what is the proper sizing for a CX frame? A road bike
I ride a 59 (long seat tube) or 60. Most all CX bikes I've seen have
been 58 or smaller.

Am I too tall for the sport? :o) Is there a rule of thumb that says
that your CX bike should be slightly smaller because of the type of
riding, jumping on and off and rough terrain?

Thanks
Frank