I'm starting a drum roaster build and was aiming to roast between 1lb. and 4lb. I have a piece of pipe that is 10-3/8" inside diameter and from my calculations, if I follow the suggested 1:1 ratio and made it 10" long I would end up with 5lb+. My concern is that 1lb would get lost in it and be very difficult/near impossible to roast. Would there be significant drawbacks it I was to cut it down to 8" or 9" long with the larger diameter?

From the past collective wisdom we have a ratio: 6 litre volume for 1 kg of greens, or bigger.

Why not start with the 10" x 10", they build in something temporary using a plexiglass front on a bench top. You could experiment with stirring vane configuration, thermocouple placement, etc; if necessary start shortening the length an inch at a time? One way to get around too much firepower is to separate the burners into two independenly controlled rows, with a center and two outside. This is how the gas train on a Feima EVO-I roaster operates.

There seem to be a lot of different ratios and formula for estimating roast capacity and I've used all the ones I've run across to get an idea of where I might end up. My question is more with the large diameter of the pipe if I make it longer 1 lb of beans would be spread out to the point there might be little mass in any one spot for things like the temp probe. If I shorten it to reduce the spread will I be introducing possibly negative dynamics like improper airflow, poor heat distribution etc? Maybe questions that can only be answered with trial and error but I wanted to check and be sure I wasn't missing something that someone had already learned didn't work well.

I would start with a longer than 1:1 first attempt, but the most important factor is the length of the burner, imo.

For the 1 kg model I am building now I am using 20 dia x 24 cm len drum, using a 18 cm long heater.
No dimension result from any engineering or design , it simply happened to find the parts with these dimensions ready to use, and will see if they match well.
Even if the ends of the drum will suffer from less heat, I don't intend to shorten the drum, but mount the two heaters shifted, to cover the whole length.

This fellow built a nice drum roaster
Home building a commercial style roaster
I built this one and it works
Another drum roaster build
I hope this helps on your build best of luck to you

Renato you hit on the importance of having everything working together properly . I'm endeavoring to do that using what I have available, just concerned that my pipe/drum may end up being bigger then I want at this time.
Jay thanks for the links. Awesome builds both of them and ones that I have gleaned ideas from.