I am not sure what you are asking.
In KivyMD MDTextField is just a line with a hint_text(If you give).
What do you mean by increasing the height of a line?
You can increase the font size of input like this.
MDTextField:
hint_text:"Enter UserName"
pos_hint:{'center_x':.5,'center_y':.5}
font_size:"36dp"
or are you looking for rectangle mode?
MDTextField:
hint_text:"Enter UserName"
mode:"rectangle"
pos_hint:{'center_x':.5,'center_y':.5}
font_size:"18dp"
or you can use:
MDTextFieldRect:
hint_text:"Enter Name"
pos_hint:{'center_x':.5,'center_y':.2}
size_hint:1,None
height:"50dp"
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