Yourssue is related to screen resolution, different devices, even if they have the same screen width and height, can have different resolutions (ie 72dpi,96dpi,etc). Font sizes are DPI dependat. The best way to ensure your text size, is to adjust your font size to your label height, measuring the "physical" size...
Take a look at the canvas object, there is a method to obtain a given string's physical height, depending on the font, and font size.
It shouldn't be to difficult to create a loop to get the desired font size