riser length question.

having built my outside mock up i now some idea of the sizes of everything. i followed the design in the book with a 45 gallon oil drum. i am still undecided about the riser diameter. if i go for 6" and want to keep the top of the drum at a reasonable height by putting the drum on the floor level that will give me about 22" of riser height. i think i need around 26" minimum. if i go for 8" i will need a lopt more height to accommodate the riser. this will bring the top of the drum too close to the ceiling the total height of the coach is around 6 foot max. i suppose what i am asking is it better to have a slightly longer 6" riser or a shorter 8" one? i need to decide this so i can order the cardboard tube for the perlite clay cast riser.

First as to whether you want a 6 or 8". It really depends on how much space you are trying to heat, and the btu's /hr it will take. Any place with little insulation (green house/shed etc) or large sq ft area, definitely go for 8".
Next, IIRC it's the ratio of burn tube to riser length (Riser at least 2x Burn and 3x Feed ), not the absolute number.