Hi ellenc,
Welcome to the Mattress Forum! 
“Euro King” is a term commonly used for the Superking, or 180cm x 200cm, or approximately 71" x 79". This can be cut down from a King (76" x 80") or California King (72" x 84") mattress in the “traditional” sizes here in North America.
Your Bultex uses 6" of CMHR (Combustion Modified High Resileince) polyfoam, at a density of 37 kg/m3, which is about a 2.5 lb density foam, which is a good quality material. The softness is rated at 3.1 kPa, which is generally regarded as “soft” with a “comparable” ILD in the low 20s, for comparison.
For special size fabricators, a good starting point would be the component supplier post here, which has offerings for latex, polyfoam and memory foam manufacturers, many who can create custom sizes for you.
Phoenix