6" is much more common than 5" that being said, it is the outside diameter of the blades when mounted, if the 6" mora blades are sticking way out from edges of the "screw" part of the auger it is a 5". Knowing all that however, had a chance to look at the HT blades in person, the only ones they had were 8" and they are made in China. You are probably better for all of the trouble to buy a new mora or eskimo 6" for around $50 with brand new blades cover etc. and be set for a long time. I've had my 6" for 10 years. A few new sets of blades, now they are now up to $30 to $40, for $20 more you get a new auger too.
Spud one, spud two, if it went through, so will you!