Hi, Arherp. Well, no one else has anted up, so I'll put my head on the block.
Ut's a rool hard question! Make a nice cup of tea, and spend an hour cruising the older threads here at BYC. You'll find numerous posts concerning the size of pens, and you'll notice that, indeed, just about the only time that acrimony ever visits us in in respect of this issue.
Now, I know that others are quietly watching me here, so:
There is probably a LEGAL minimum size for your district ask your local council.
There is a size that a professional breeder sees as sufficient she's profit-oriented, and is concerned with the general health of her flock, not with any need that an emu might have to feel the wind in its hair (feathers).
There's a size that an amateur breeder, a bird LOVER, might feel gives the birds an environment that replicates to some degree a natural environment, in which emus will diddley bop mile upon mile.
There's me (whom I suspect some readers haven't quite worked out: I am a hard-core political animal, retired in ill health, with a broad knowledge of revolutionist politics, including even those of the capital-P-people-of-whom-we-speak-not, whom I personally suspect are all wearing underpants a couple of sizes too small), who could take snide shots at all and sundry by pointing out that MY emus have an entire CONTINENT to run around on -- though they'd best not be caught in the canola crop of the guy next door, lest they have a run in with a high velocity reprimand.
Cup of tea. Read the older posts.
Supreme Emu
Rocky Gully, Western Australia