How can I use 这么/那么 properly?
[这么 + adjective] = [this + adjective] referring to something close by

[那么 + adjective] = [that + adjective] referring to something that is not close by


你这么高，means "you are this tall" (talk about here and now) 
你那么高，means "you are that tall" (talk about a situation not in the present) 


For example, the reason you were accepted into the basketball team is because you were that tall 


你这个男朋友是不是矮了点儿? "Isn't this boyfriend of yours a little bit too short?" (the boyfriend is either right there with them or they are referring him as the object of discussion in front of them)
你那个男朋友是不是矮了点儿? "Isn't that boyfriend of yours a little bit too short?" (the boyfriend is not right there with them)


'this tall'  and 'that tall' can be replaced by 'so tall'