Top screws are 20 mm, middle one is 10 mm. These have thread lock.
Upper left screw is 4.5 mm and has smaller head. The other four are 6 mm.
This photo is a little confusing. The shield should be off, which would show the correct positions of the yellow and orange screws. (The orange circle is in the wrong place.) Furthermore, these screws don't need to be removed at this stage (they hold the logic board to the frame). And finally, on my 800 MHz machine, BOTH these screws are 6 mm in length.
If you have been following along, the 6 mm (orange) screw here would have already been removed in step 24, where its position is not quite correct, and it is incorrectly labeled as a "4.5 mm" screw.