How do I determine the sizing for a new RFI issued helmet (IHPS)?

People have oddly shaped heads. There are only two or three sizes available in the helmets. It should fit with all the pads correctly inserted in the correct configuration. That means the crown pad inserted, the trapezoid pads in the front and back, and the four oval pads standing vertical and covering the bolts, all flush with the lip of the helmet. If you have to remove a pad for the helmet to fit correctly, then the helmet is too small.

So after what seemed like a lot of digging, the helmet sizing shouldn't have more than a 1" space between your melon and the helmets inner surface after you put the suspension pads in (remarkably the pads are about 1" thick) so if its flopping around and the pads are not touching your melon on your temples, forehead or back of your head, it's to big. If the pads are compressed tightly on your temples it's to small. The helmet should have come with either a seven or nine pad suspension system depending on the size.
You basically have to move the pads around until it feels rite like the old ACH. For more info go to the 3M IHPS site and look at the model F70 product brochure. Also your helmet should have come with a care and use manual so if you didn't get one with it see if one of your buddies got one with theirs.

I am not sure what the real rule is, but typically there is about an inch at the widest part of the helmet with a gap before it hits your noggin.
Rearrange your pads or add pads to make it comfortable.
You can snag a cheap set of unused pads ar pretty much any surplus store, or get a hookup from supply.