Sizemore normally refers to Wanda as "Lady Firebaugh", and seems to respect her. He does not like having to defend her actions to the casters of the Magic Kingdom. As a caster, Wanda is probably aware of Sizemore's "Rock Star" status in the Magic Kingdom. She is at least superficially respectful toward Sizemore, but can be inconsiderate and curt, as she was when Sizemore suggested that Jillian could break Wanda's Suggestion Spell. They have a functional working relationship but do not appear to be social. In particular, Sizemore's willingness to study fields of magic he has no ability at puzzles the multitalented but specialized Wanda to no end.
Wanda may have gained a greater appreciation of Sizemore as a result of linking with him to uncroak the Volcano. If Wanda believes that Fate requires that other members of her side attune to the other Arkentools, she may see Sizemore as a likely candidate for this role.