From looking around the net I found that in the "Naval Service Manual 381 - U-Boat Training for U-Boat Type VII C"
attack periscope above keel = 14663 mm
search periscope above keel = 11999 mm
Can easily be changed.
Here's how :-
First make a back up of your Data folder.
Using goblinEditor open the file "UBoot_Sensors.GR2" data\objects\sensors
Then choose merge from the file menu.
select "UBoot_sensors.sim" data\objects\sensors
Then in the project tree double click on "obj_Extensible @ atack_periscop_wire"
Click on obj_Extensible
And change the max height (It was at 8 meters. I changed mine to 4.6 meters).
Save.
Pics of how scopes look fully raised with attack scope setting at 4.6 meters