I think "big" is relative to the relationship. If you're Bill Gates, you can definitely buy a $50,000 car without talking to Melinda about it first, but you'd definitely want to consult her before starting a new company that has her name on it.
If you're Jim, it's pretty un-cool to buy your childhood home to be your new house without consulting Pam. Which is what he did.
If you're dating, applying for and accepting a new job that moves you two hours away from your partner would count as big.
I'd basically say, anything that has the potential to drastically change the lifestyle/financial situation of both you and your partner would count as "big".