The official WeatherFlow pole mount is designed for two sizes: 1.315 inches / 33.4 mm and 1.25 inches / 32 mm. And any pipe/pole/mast within a mm or so of those dimensions will probably work.
Why the odd sizes? Well, 33.4 mm is the outer diameter of what turns out to be a pretty common pipe size all over the world. It’s what Americans who work with such things would call “one inch pipe”. The official size is a north american standard known as “NPS-1” (aka “1 inch Nominal Pipe Size”). It’s also a common pipe size in the rest of the world, officially called “DN-25” (aka “25 mm diamètre nominal”). Despite the official label, the outer diameter of such a pipe is neither 1 inch nor 25 mm in diameter, for reasons that are outside the scope of this forum. 
The 1.25 inch / 32 mm size is another “standard” - it’s a common size for antenna mounts, which folks often use for mounting weather stations.
This and other SKY mounting tips can be found on the Proper Siting & Installation section of our support website.