Both ASME B16.10 and API 6D/ISO 14313 define face-to-face for up to NPS 12, Class 2500, which for Group 1.1 materials, is good for 6170 psig (425 barg). So you can use API 6D for the lower pressures.
For 10K, 15K and 20K valves, there are no international standards defining F/F or E/E length. Most end-users expect "manufacturer standard" in this case. Additionally, since there are no standards in the larger bore sizes for these pressures, and due to the weight involved, most end-users will specify the use of either Weld-End or compact hub connections such as Grayloc, Taper-Lok, or Norsok L-005. In specifying a WE valve, there is no need for a standard E/E length, because the piping will just be butting direct to valve anyway, and if the valve needs to be replaced, an entire section of piping will be cut out for the replacement. When using a hub type connection, the design is generally looking for additional weight-savings, so they are as short as the manufacturer can make them...again, to their own standard.
Hope that helps.