You can't choose to spend more mana to cast Wurmquake than you're asked to just to make a bigger Wurm. In most cases, it's a 6/6 Phyrexian Wurm the first time, and a 10/10 Phyrexian Wurm when you cast Wurmquake with flashback. However, if there are any additional costs to cast it, the mana spent to pay those costs counts.
Wurmquake
A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Xavier Ribeiro
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
