How many character can it name of global variables? (max. enable long name)

Never seen a limit published. I just tried one with 402 characters, and it worked fine.

Why don't you experiment and report back if you find a limit? 

a) GlobalVariableSet("123456789_123456789_123456789_123456789_123456789_123456789_xxxx",0);

result: glob. var:  123456789_123456789_123456789_123456789_123456789_123456789_xx value 0,  I cannot delete key F3, I have to use mql code GlobalVariableAll(); 

b) GlobalVariableSet("123456789_123456789_123456789_123456789_123456789_123456789_1x",0); 

 result: glob. var:  123456789_123456789_123456789_123456789_123456789_123456789_1x value 0, I can delete key F3 and delete button

on my pc (XP, build 765):

Interesting - you're quite right that my 402 character GV could not be deleted from F3 (I was working entirely in code).

Might be worth mentioning it to the service desk.