SKNode.hframe
でわかるように、プロパティは読み取り専用です。
@property (SK_NONATOMIC_IOSONLY, readonly) CGRect frame;
もう少しSKNode
ドキュメントを読むことをお勧めします: https://developer.apple.com/library/ios/documentation/SpriteKit/Reference/SKNode_Ref/Reference/Reference.html
ドキュメントには、次の段落があります。
The frame property provides the bounding rectangle for a node’s visual content, modified
by the scale and rotation properties. The frame is non-empty if the node’s class draws
content. Each node subclass determines the size of this content differently. In some
subclasses, the size of the node’s content is declared explicitly, such as in the
SKSpriteNode class. In other subclasses, the content size is calculated implicitly by the
class using other object properties. For example, a SKLabelNode object determines its
content size using the label’s message text and font characteristics.
したがってframe
、さまざまなプロパティに基づいて SKNode サブクラスによって計算されます