Choosing icons dimensions for Smartphone App

Hello ,
I am new to Unity . I am developing an App which can run on Android. This app is like a camera app with some control icons on the camera Over lay.
I have taken a Canvas on which i am putting the webcam texture and put some buttons on it .
SO what i did is I downloaded some icons from google and sliced them and made sprite assets from them and then put them on the buttons.
I have taken png files with 24X24 or 48X48 size .
But I am seeing the icons are stretched out while running on Windows PC and while running on Mobile.
What is the best approach we can use and from where we can get the free assets to be used?
I also wants to add some transparency to the buttons so that it do not block the view.
Please advice.

This question is not under the right area. If you post it under a UI related area (for example UGUI TextMesh Pro), they may have more context and expertise to answer your question.