When attempting to cook bone broths, what is the correct technique of breaking up long bones which do not fit inside the crock pot?   I can think of hammering or sawing off hand, but both seem to be rather messy techniques. Is there any good method to break off large bones (relatively) safely? 
A hacksaw (used for cutting metal) is a relatively inexpensive hand tool.  Keep a blade set aside for food use only, or buy a new one if you don't use it frequently for that purpose.  That's good for more even, straight bones.  I'm not sure how that would work on a knuckle bone like others are talking about.  A farmer at the outdoor market had a beef femur that he sold me for $3 (whether he should bring it and try to sell it was obviously the cause of some family disagreement/discussion, as his pre-teen son freaked out when he heard that his dad actually sold the darn thing).  I used the hacksaw to get it into the stockpot.  When I was done making stock from it, I hung the two halves from the tree in our front yard as part of Halloween decorations. 