how big is this coking resource

Just a roughly figure:
Assume this hard coking coal density is 1400kg/m3=1.4ton/m3
1500h with 1.7m thickness:
15,000,000m2 x 1.7m= 25.5 mil m3

25.5mi x1.4=35.7 mil ton

50% x 35.7=17.8 Mt

That's the maximum, It's 100% of 1500h with 1.7m thick. I think it's better put a factor 0.7 (assume coking coal exist in 70% of this area)

17.8Mt x 0.7=12.5 Mt.