I am not particularly familiar with the Howard Leights as I have been using Sordin/MSA and TCI for many years. Are you planning to run them with an OTH [over-the-head] headband, or using some form of swing-up rail mounted adapter?
My experience with a borrowed TW is that OTH headsets do not work well because the helmet suspension and headband interfere. With my OCs, I run the headband BETWEEN the suspension and the helmet shell. With the MICH, I use a split crown pad to provide clearance for the headband. With a Nightcap, the headset obviously fits over the top, but may not provide the best of seal over your ears depending on how low the cap rides on your head.
As far as melon size, do you have a seamstress tape measure? If not, you can use a piece of string or whatever and a regular tape measure, yardstick, etc. Measure around your head at the widest point above the ears. If you know you hat size, take that number and multiply by PI [3.14159].
ETA - I just noticed that you are HFD.