I created one task, where I have white background and black digits.

I need to take the largest by thickness digit. I have made my picture bw, recognized all symbols, but I don't understand, how to scale thickness. I have tried arcLength(contours), but it gave me the largest by size. I have tried morphological operations, but as I undestood, it helps to remove noises and another mistakes in picture, right? And I had a thought to check the distance between neighbour points of contours, but then I thought that it would be hard because of not exact and clear form of symbols(I draw tnem on paint). So, that's all Ideas, that I had. Can you help me in this question by telling names of themes in Comp. vision and OpenCV, that could help me to solve this task? I don't need exact algorithm of solution, only themes. And if that's not OpenCV task, so which is? What library? Should I learn some pack of themes and basics before the solution of my task?