I am asked to bid on a 21000 sq. ft.two story building plus six smaller buildings, plus mileage of about one half hour drive.

Bid for what ? And why charge them travelling time ?

Bids to buy? Renovate? Clean? Paint? Exorcism?

Might be pretty expensive to buy. DO they come with the refinery or the oil wells? And do they all work.

I'm guessing cleaning. You seem very naïve about all this so it's going to be WORK.
Square feet is just the beginning.
Make a list of ALL the requirements, in detail, that they want or might want, put in a clipboard, and go around with someone there to discuss each item and have them sign it.
Go home and estimate and then add at least 20% to it, because they always want more than they say.

@joy. Yes my guess was cleaning too but if you have to ask how for something that magnitude, you aren't even in the league to do a job like that. And on top of that charging for a 1/2 hour drive !