Bike boxes come in sizes. Since you are packing a road bike, you are going to want a larger size box. If you are scrounging a bike box from the bike shop, ask to look inside at a factory packed bike. EVERY LITTLE DETAIL COUNTS so don't skimp on anything.
Here's a hint. Remove the front wheel and package up the bike outside of the box. Then stand the box up on one end and set the bike into the box on it's rear wheel.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.