Hi Group 
Re the Above
Here in NSW when I was Colliery Manager, there were no standards as such, mine timber including props was ordered by individual mines (even if part of a group).  
 
Tenders were called (probably every two years) for the supply of mine timber with manager specifying the sizes (which he determined by the seam conditions)  Being natural grown timber this referred to a minimum "small end" diameter.
 
In the mines under my charge seam heights ranged from 2 to 36 feet and in general when setting out timber contracts I followed the sizes that the mine had used over the years.  (if it's working don't meddle!)
 
So, say we were working a six foot seam the contract would call for the supply of props six feet long with a small end diameter of (say) six inches. The "timber getter" could supply larger diameter timber but was paid no more, 
The mine could reject any undersized timber though one had to be careful and have it removed from the property by the contractor or one's supply dried up ("Very boggy in the bush Boss!") till the pit ran short of timber and the rejected props and legs were by necessity taken underground.  
 
When set they would be cut at the face to exact length to allow for a "wedge" and "lid" against the roof.
 
Of course the Undermanager kept an eye on things from out end and if there was too much being cut off the face timber (soon checked up by the block ends) he would ask for shorter props (in six inch increments) with the next fortnightly order. 
 
And if a load of timber was supplied too large in diameter (with consquent grizzles from facemen)  he would not send it back but roll it to the back of the heap for the day when bad roof required extra strength.

Also specified in the agreement was the species of timber acceptable:- eg spotted gum, ironbark, string bark, etc (all hardwoods of the eucalyptus family) and the quantities required.
   
Vertical timbers with no cross members  were "props"; when supporting cross timber they became "legs" 
Cross timber  was known as "slabs"  (flat and 6 to 12 ft); "baulks" (round and 16 to 24 ft) and "stringers" (round 12 to 30 ft and non-loadbearing..to support of cables, brattice etc.) 

In the mines I managed, seam heights ranged from 2 to 36 ft (vertical timber set to this height was of massive dimensions)

Regards 
John