perlite

The formula for diameter to circumference is pi*diameter... 3.14159  times diameter, so for 8" pipe
3.14159 • 8 =  ~25.133 inches

So 25-1/4" should be enough....  25-1/2" may leave you wanting to trim it.

I realized I trimmed mine to just a tiny bit more than the circumference if my pipe, and it was compression of the inner diameter of my blanket when rolled up in the pipe, that holds it in place.

 The inner diameter of the cfb wants to be pi*6, and is cut at pi•8, so no significant overage is necessary.

It may have "ripples" running up the inside from that 25" of blanket being compressed to the less than 19" at the inner 6" circumference... Mine did.  Works fine.

Thanks Matthew, that's a big help:)

What's the best height for the heat riser?

I did some more checking and Evans Ceramic Supply Store in Wichita, Kansas is the only store that carries Lincoln 60 Fireclay in my area, and they also carry the ceramic fiber roll which they cut to size and charge $6 per sq ft. So how many bags of the fireclay will I need if I use it for my cob mix instead of harvesting the clay myself? The more sacks of clay I purchase the cheaper the bag. If I buy 100#s it will cost me $30, that's $15 a bag. I'm going Tuesday to get it.

Loretta,      Staci Kopcha https://permies.com/t/73081/RMH-Dummies-guide-build is currently building a RMH with a bench and is using fireclay....Perhaps she could provide some info for you.

Hi Loretta;   How much clay you will need depends on how you are planning to build your mass.  If you plan on a traditional exposed cob bench (like Stacie) you will use quite a bit. If you plan on a covered bench  you will need much less and it can be dirty.
 
Did you have a plan in mind ?