Quick question that I couldn't find clarification for in the rules book/PAC:

Can a character (that flies) carry a multi-based figure that has the symbol?

Specifically, can XDPS G019 Vange Whedon (who has a four by four base) be carried by a single-based flyer when Vange Whedon has this special power: HUMAN UNTIL SPLASHED BY BLOOD...: Vange Whedon is [Standard symbol] instead of [Colossal symbol]. When Vange Whedon or an adjacent friendly character would take damage, they don't take that damage and instead turn Vange Whedon to her starting click. Protected: Pulse Wave, Outwit.

Can carry

(unless there's some special Galactus rules I don't know about, but the concept is the same)
Base doesn't matter, standard size doesn't matter. Anyone can generally carry a smaller character, and anyone with Passenger:X can carry and equal sized character without flight.

In short, the answer is yes, she can be carried by another flier on that first click.

I don't think that's right? Unless there's something I'm missing, she's still a flier on her human click, so she can only be carried by a larger character (or a character that's allowed to carry fliers).

She has Earthbound on that click.

Earthbound does not remove Flight anymore. All it does is turn off Improved Movement, Improved Targeting, and Willpower. Symbols all stay the same. The ability of Flight still remains, so as weird as it is, a flier with EB/N can carry another figure, but will get caught in the bushes, can't change elevation, etc.
As for the question, there is no restriction on the base size for carrying. All that matters is whether the character size is smaller, or if the same size, whether the one being carried has the Wing symbol.

Keep in mind that while multi-based figures can be carried, they can't be TKed.

TELEKINESIS POWER: Minimum range value 6, Improved Targeting OO-->. Pick up an object (light or heavy) within range and line of fire, then give a RANGE Object Action at no cost or place it in a square within range and line of fire. //
POWER: Minimum range value 6. Place one target single-base character within range and line of fire into another square within range and line of fire. That square must be within 6 squares and line of fire from the target’s current square. If the target is an opposing character, first make a range attack and the hit character is then placed instead of normal damage. Characters placed by this power can’t use Telekinesis this turn.

Just so we’re covering all bases, also note that the flight-symbol restriction is only a part of Passenger: X, not the inherent Carry ability. This means a larger character ( or ) can carry a human-click Vange Whedon