Default: when designing a website, make every single button the same size, unless there's a really really good reason.
This is actually a bad foundation for designing a site. The chances that every button is going to be of equal value in a given setting is close to zero. The default ought to be: when designing a website, assume that important buttons will be bigger than unimportant ones, unless there's a good reason.
Another example: When dealing with a customer, assume that unless there is good evidence to the contrary, they are trying to rip you off.
Another example: Assume, all other things being equal, that next year will be just like this year.
And a final default: Assume that unless they've notified you otherwise, all prospects are willing to get spam (press releases, phone calls, letters, ads) from you.