I am Building a app using Kivy , KivyMD in Python and I face a problem , the size of the text that I set for a Label is much bigger to a small devices and very small for a bigger devices like Tablet is that possible to set the size of a text dynamically ?
is that possible to set the size of a text dynamically ?

Yes, just set the font_size property of the Label to what you want, when you want.
It sounds like you actually want to know if you can you set it to be constant between different devices. The way to do this is to use device-independent pixels - instead of font_size: 12 use font_size: dp(12) in kv (12 is just an example), or in Python code do the same thing but you'll need to import dp from kivy.metrics.
Is that possible to set the size of a text dynamically in Python Kivy App?