Brass 3/4 inch BSP BS1212/1 part 1 float valve (also known as a ball-cock, inlet valve, fill valve or ball valve).
This size of ballcock valve is more usually found in an industrial environment. You are very unlikely to have this in your house (unless you live in a really huge house). Most houses use a 1/2 inch valve.
Threaded ball mounting end has 5/16 inch thread - use a 6 inch float.
** BSP thread sizes are measured a special way - have a look at our information page for more details **