I have a 2013 tues pro with the bos fork and need to replace my headset but I am struggling to find many details about the sizing and failing to understand the details that I can find! I have found that the current headset fitted is an Acros azx-205 zs56,zs56 for my year on a bike review and from the yt website for the current 2015 model it says an acros azx-218 zs56,zs56.

Not sure which one to believe or what size that makes them!

Right, the crucial numbers and lettering is the ZS56.
Ignore the Acros azx stuff, that’s just Acros’s in house model designation.

ZS56 means: zero stack 56mm, in other words an integrated/internal cups headset in 56mm steerer variety (which is I believe a 1.5 inch steerer).
The fact that both top and bottom cups are listed as ZS56 implies you have a straight 1.5 steerer, a tapered steerer would be ZS44 ZS56 (1.125 & 1.5).

Go on CRC and pick a Nukeproof Warhead headset that is listed as 56IISS (56mm/1.5 inch steerer, Internal top cup, Internal bottom cup, straight steerer).

Great stuff, thank you