To me it seems like he can avoid at least some of the refinement slots for a focus item since he can't carry it around. This could act as either a discount in refresh or he could treat it as an upgrade to his workspace giving him a +X to Lore when using his workspace for a divination in Chicago.
Actually this is factored in by the table on YS 281.
A 5-6 slot focus is basketball sized. In fact, the marginalia on the same page talks about focus items in excess of 6 being "vehicular sized" (i.e. a 7 slot focus is the size of a smart car). And Dresden responds "There's no Little Chicago in my pocket" (he must be happy to see us then).
But that is just the total slots. So it might be +4 Complexity and +4 Control on Divination. But knowing Dresden, he would have as much Complexity as possible, and not bother with control (just take longer to start the spell going).
However, since Dresden is constantly making skill checks to maintain Little Chicago to keep it current, those act as extra +2's for the complexity.
-Resources: Equipment.
-Endurance: Long-Term Activity (going around town chipping buildings and grabbing bark samples)
-Craftsmanship: Building.
With Lore 6, 6 focus slots, and those 3 skills, his divination start at on the fly of 18. Although I think by the rules, 6 of that can only be tagged once (then you have to invoke with fate points).
The Extended Divination is detailed on YS 297.
Base Complexity: Target's Conviction + 4 + threshold/wards. Duration is 1 shift per step up time scale (starting from a scene for 0 shifts). I think a scene is about 15 minutes. So spying on a conviction 3 character for a day that is sitting in his apartment is 13 shifts (3 + 4 + 1 threshold + 5 duration). Of course if you just wanted to spy on the apartment, you could scrap paint from the door and spy on the place for 10 shifts (but then you can't follow him if he leaves the location).
... And this gives me a scenario idea.
