The box on targets and sizes says: Intellego spells are not affected by Target size.
Can I target a size +10000… group of images then and see all of Mythic Europe?
I thought sailors foretaste of the marrow had a size magnitude but that seems to have changed in the definitive edition.
You are referring to the size of the Group. That sidebar is about the size of the Target. Specifically, to the base Individual. It even describes how to calculate that in the paragraph about Groups.
Furthermore, for a measly Group ×10000 you’re going to be hard pressed to define a Group that is sufficiently ubiquitous but small enough in numbers to be useful for that. Maybe ants.
Size as in dimensions (like the height and weight of a person/+2 size, say, vs +0), not quantity.
Yes, and that does indeed allow some impressive Intellego effects.
I'd note that a +10000 Group is a Group which would have 10000 magnitudes tacked on for size, i.e. a group that's not x 10000, but a Group whose size is x 1-followed-by-10thousand-zeros that of an Individual. Just to put things into (modern) context if you assumed your based Individual was a single atom, then +100 (not +10000) magnitudes would vastly exceed the total number of atoms in the universe.
I think the issue here is a different one: can a measly human, even one enhanced by magic, simultaneously "process" the whole of Mythic Europe if given access to all of it from a gazillion points of view? I'd rule against that.
Thanks, so I was on the right track. I did indeed check the number of atoms in our universe and added a few zeros to be on the safe side 
I’ll be invoking the central rule then and perhaps limit myself to “processable” free size magnitudes roughly equivalent to Int, at least when things seem to get out of hand.
That way a very smart (+5 Int) character can try to search a whole village/small town* without getting overwhelmed, while a lesser individual has to make concentration checks every round to avoid that. Larger group sizes are beyond mere mortals.
You also just get access to the images from one point of view that you can shift like in prying eyes. And standard Group rules apply, the images need to form a reasonably separate group from the sea of images around them.
*Cylinder of 500 m diameter and 5 m height, 10% of which is occupied volume, is equivalent to +5 size group of people, estimated to be 1e6 blocks 1.75 m high, 25 cm wide and 20 cm deep.
((500m/2)^2 *pi *5m *0.1)/(1.75m *25cm *20cm) = 1.121e6
I think the point of the “intellego spells are not affected by target size” is that a spell to locate an immense dragon based on its droppings requires no more magnitudes than a spell to locate a mouse from its droppings. The thing you’re trying to extract information from, if it is a singular thing, will still be an individual target. If you’re trying to gather information about an area, then you’ll need a suitable target (room/structure/boundary) and that complicates things.
The reason I asked about Group as a Target is because I was specifically thinking about Intellego Imaginem. With any other form, a Group would typically not cover everything in an area like a town. (If you don’t happen to investigate one built on bare rock, out of rock, by rock people, eating rocks with rock utensils etc.)
But everything has an image, so it’s much easier to find a valid Group target that covers a huge area. Still, I imagine it could sometimes be useful to target a very large group of e.g. herbam individuals to investigate a whole forest.
I'd rather use Perception for what you can gather from the spell. This is about finding Waldo and not crushing maths. (well, that's the excuse I found because Int is overused.