TubeNet

As an amateur low brass player, now mostly tuba, I have always been unsure of what horn size standards are. I see 6//4, 4/4/ etc, and don't know what determines size. For example, I have a Boosey and Hawkes Imperial, which I have assumed would be 4/4 , but don't know for sure. Is there an easily understood standard? If so what determines size?

Those designations can largely be ignored.

They're more like guidelines. Most tubas can be defined as one size, but it is not standardized. For example, Rudy tubas are generally considered to be a size larger than what Rudy calls them.

They’re whatever the marketing division of a tuba manufacturer wants them they are.
Or whatever the TubeNet Freak Jury decides them to be.
There never has been ANY standard...

The master, Rick Denney, suggested that the designation of 3/4, 4/4, 5/4, 6/4...etc could best be standardized by measuring by internal volume of the outer branches of the horn (perhaps by filling with water, and then measuring the volume, that way!).

Having said that, traditionally the designations have been just guestimates.

Bottom line is: it doesn’t really matter what you call it...it’s a TUBA!

Two of the best known tubas that are considered to be "4/4" tubas are the Miraphone 186 and the King 2341 (new number). Any tuba that is about the same size is generally considered a 4/4 tuba. Anything smaller, like the Miraphone 184 or King 1135, are generally considered 3/4 tubas. Anything larger, like the Miraphone 187 & 188, are considered large 4/4 tubas or 5/4 tubas, depending on the opinion. Anything larger than that, for example the York Chicago C tubas, B&S Neptune, etc., are generally regarded as 6/4 tubas.

It is all very informal and open to discussion. For example, as mentioned, the Miraphone 187 BBb and the 188 CC are called a large 4/4 by some folks, and a 5/4 by other folks. Then there is Rudy Meinl, which does label each of their sizes of tubas with a */4 designation, although what they call a 3/4 BBb has a 17-inch bell and is essentially the same size as a Miraphone 186, which we consider a 4/4 tuba. This points to just how inconsistent it is to call a tuba by a */4 size