Spinoza addresses this topic at length in his Note to Proposition 15, "Whatever is, is in God, and nothing can exist or be conceived without God."

But, HOW can God be considered an extended thing since most philosophical schools deny that God could have a body. And no physical substance means no extension, right? — SapereAude

For Spinoza God and Nature are the same. They are all inclusive terms. It is not that God has a body but that extension is an attribute of God.

Spinoza cannot be understood as long as one conceives of God according to the "schools".

Thanks for your response. What "schools" are you referring to precisely?

It was you who mentioned schools:

But, HOW can God be considered an extended thing since most philosophical schools deny that God could have a body. And no physical substance means no extension, right? — SapereAude

But, HOW can God be considered an extended thing since most philosophical schools deny that God could have a body. And no physical substance means no extension, right? — SapereAude

Spinoza has two notions which are similar to 'property', so when we're saying something 'has' extension you have to be careful whether that 'has' predicates in the sense of Spinoza's attributes or Spinoza's modes. Attributes apply specifically to substances, modes apply to everything else.

A substance 'has' extension in the same sense (roughly) that a triangle has three sides. The structure (having three sides) is a necessary feature of the thing (triangle). In Spinoza's terms, extension is an attribute of substance. As it says in definition (IV):

IV. By attribute, I mean that which the intellect perceives as constituting the essence of substance. — Spinoza

An entity 'has' extension in the same sense (roughly) as a person has a personality which is their own; it's (personality) a contingent feature of the entity (person) shaped by interactions (modifications, conceived through other than itself). As it says in definition (V):

V. By mode, I mean the modifications of substance, or that which exists in, and is conceived through, something other than itself. — Spinoza

So, if entities are extended, substance's extension is the sine qua non of their extension.

So I cannot figure out how Spinoza comes to the conclusion that God has infinite extension.

Then we need to look at why for Spinoza, substance's extension is infinite. Infinity in Spinoza is bound up with the notion of non-limitation. The first time this crops up is in definition (II), but non-limitation is at work 'in absentia' so to speak:

II. A thing is called finite after its kind, when it can be limited by another thing of the same nature; for instance, a body is called finite because we always conceive another greater body. So, also, a thought is limited by another thought, but a body is not limited by thought, nor a thought by body. — Spinoza

Imagine a pie. You cut the pie in two, the amount in each piece is limited by the amount in the whole pie. Thus, the pieces are finite (in extension) after their kind. But also remember that the first line of the definition says '...when it can be limited...', not necessarily just that it is. So the pie pieces are finite after their kind because they were capable of being limited, not because they actually were.

For this division of the pie to make sense, and also for the division to come to exist, there has to be an underlying extension which both pie pieces and the whole pie partake in which can lend reality to the extension of the pie and the pieces; that underlying extension (prior to all pies and pie modification) is the extension of substance. As an attribute (extension itself), not a mere mode (pie piece sizes).

The second time this logic crops up it does so explicitly, in proposition VIII:

PROP. VIII. Every substance is necessarily infinite.

Proof.—There can only be one substance with an identical attribute, and existence follows from its nature (Prop. vii.); its nature, therefore, involves existence, either as finite or infinite. It does not exist as finite, for (by Def. ii.) it would then be limited by something else of the same kind, which would also necessarily exist (Prop. vii.); and there would be two substances with an identical attribute, which is absurd (Prop. v.). It therefore exists as infinite. Q.E.D.

The logic here is a disjunctive syllogism. X is either finite or infinite, can't be finite, so it must be infinite. The reason the finitude of substance fails for Spinoza is that substance itself 'contains' no (de)limitations, there are no properties or entities which cease to be 'active' for substance at any time. If substance had a bounded extension in space (has in the sense of attribute), it would not be infinite firstly because its extension is limited but more importantly because this requires applying any limitation to substance which contradicts the 'infinity' of specific attributes (like extension) set up in definition VI:

VI. By God, I mean a being absolutely infinite—that is, a substance consisting in infinite attributes, of which each expresses eternal and infinite essentiality. — Spinoza

However, we have to remember that substance's infinity isn't an infinity of a specific sort - like infinite extension, but the absence of delimitation tout court (specific sorts would also have a logic of delimitation!). So for a concluding remark, I'll share the mental picture I have of Spinoza's idea of god or substance. Imagine we have an exhaustive list of every possible way of being, every possible way for which something could be and is. Substance is that which has all those properties (attributes) always and forever. Substance is the cosmic light switch which is always on for every way of being, and the light controlled by the switch.

That is an excellent explanation. Great metaphor.