To understand extension I do not believe you need to understand his idea of geometric being (or objects as conceived geometrically). Geometric being would be an attribute as perceived by intellect or mind, as a mental aid in measure. Or to use his terminology, as an "imagining" and not a comprehension (understanding the composition of relations constituting a body). Thus one is not in the same plane of discussion as with simple bodies in extension. One would be thinking in images of existence instead of being in relation to the existent. It sounds minor but he wrests a lot on establishing a sort of isomorphic non-causal interaction between these two separate planes of modality.
He wrestles the most and fleshes out his thinking on this topic mostly in the Treatise on the Intellect.
Anyhow, starting from the beginning of the Ethics:
Proposition 1 establishes that God thinks; and
Proposition 2 establishes that God exists in extension.
then, this parallelism runs throughout all attributes of substance. Bodies exist. Thinking exists. All deriving from God as modes of extension and modes of thinking. And more importantly, this parallelism exists in complex relationships of reciprocity (e.g. "The mind is therefore the idea of the corresponding body" Book II Prop 11)
All bodies thus refer to the other finite existing modes that determines it. Put another way, all extension is determined by its extensive parts/relations. Any type of body presupposes an existence which exists in extension and which can be affected by and can affect other bodies.
I think there is the extent to which he postulates on the quality of extension-in-and-of-itself. The Kelvin knot type of idea is interesting but I don't think he would see that type of qualification as necessary for defining the essential character of extensionality, which he aligned more to the idea of Power of action (i.e. what it could do in affectus, affectio or essence).
Measures (such as geometrization), numbers, signs, images are all occurring in mind or thinking, not in extension itself.