Whether Ukraine has "control" of territory is deferred to credible sources (e.g. Critical Threats, BBC).

"any part" means any single location. If Ukrainian forces make a beachhead via naval landing on the Crimean coast and hold it for a week, this would count. If Ukrainian forces control a farmhouse north of Perekop on the Crimean side of the boundary, this would count.

"at least a week" means continuously holding that single location. If Ukrainian forces hold onto a single location discontinuously (have it one day, then lose it the next), or hold onto different locations for a continuous week, neither of these would count.