The size property has probably just not been exposed in GDScript. Almost every GDScript property actually calls getter and setter functions internally though, so functionally using get_size or just size is the same thing.
For example, the following are equivalent, just one uses the property interface bound in C++ while the other uses the getter directly:
# internally, these are the same! C++ maps properties to getter/setter functions.
var prop_trans = Spatial_Node_Here.global_transform
var getter_trans = Spatial_Node_Here.get_global_transform()
