This happened to me when I used ui .View.present(self, sheet') in ui.View._init_().
def __init__(self): # ... self.present('sheet')
At this time the UI elements are not initialized and the size is 100 x 100. I moved it to ui.View.did_load(self) and it worked out alright.
def did_load(self): # ... self.present('sheet')
Thanks to the wonderful debugger in pythonista 3!