Is it remotely feasible to have a generic green common fatty in gold, with flying? I fear not; I fear this would have the same board impact as a Shivan Dragon and completely distort Limited.

What size can a common gold flyer sensibly go up to?

Good point. 3/3 is definitely ok. Errant Ephemeron is the only 4/4 I can find, but suggests 4/4 is ok if costed appropriately. I fear you're right about anything larger than that.

Possibly this would be a place to use our intermittently-flying technology? A 5/5 which flies depending on something, or can be blocked by some non-flying creatures?

As it turns out, most of the gold commons are creatures; I think this common slot will be needed for a noncreature spell. A creature of this size is much less of a problem at uncommon (see Serra Angel), so I'll let this turn into a generic fat uncommon flyer at about 4/4, and leave the question of a common gold flyer's maximum size unsolved for now.

My first thought lined up with Jack's second. We may want to add the occasional large flyer in common to this set because the set can handle it. We're looking at at least 5 anti-flying cards in common... if a large flying common could be in any set, it would be this one.

I was hoping this would be a reference to Beast of Burden somehow. If we want to have an arbitrarily large flyer, a rare flying version of the original could be cool.

Like a gold Battle Squadron? Yeah, seems reasonable. But no, this was concepted as a literal beast of burden, like an ox or something, but used in the flying city.

Flying buffalo. The wings are the best eating.

I like the idea of a nice simple "This is a big flyer. Very big." card.