This answer depends on the size of the Durso Standpipe you're interested in:
For 3/4 Inch Bulkheads:
A Durso Standpipes designed for 3/4 inch bulkhead uses 3/4 inch schedule 40 PVC for the bulkhead adapter. The rest of the standpipe is made from 1 inch schedule 40 PVC and schedule 40 PVC fittings. The fittings are cut down to make the unit as compact as possible. Schedule 26 PVC pipe is not available to me in 1 inch size.
For 1 Inch Bulkheads:
A Durso Standpipe designed for 1 inch bulkhead uses 1 inch schedule 40 PVC for the bulkhead adapter. The rest of the standpipes uses 1.25 inch schedule 26 thin-wall PVC pipe and 1.25 inch schedule 40 PVC fittings.
For 1.5 Inch Bulkheads:
A Durso Standpipe designed for 1.5 inch bulkhead uses 1.5 inch schedule 26 pipe for the bulkhead adapter, which reduces down to 1.25 inch schedule 26 PVC pipe and 1.25 inch schedule 40 PVC fittings.
A Durso Standpipe designed for 1.5 inch bulkhead uses 1.5 inch schedule 26 PVC pipe throughout with 1.5 inch schedule 40 PVC fittings. This standpipe has no reducers.