Big ideas have to at least "pencil" on the drawing board in order to work.

An idea that pencils is plausibly viable. That means it might work.

If you can't get a big idea to even pencil, then it definitely doesn't work.

The real world is much less forgiving than the drawing table.

This is only true for ideas that are already big, where the question is the downside of how likely it is that it doesn't work.

For ideas that start small but might grow into something much bigger, the value you're able to figure out on the drawing board will undercount the upside.