Flutter Measurable Grid

Hi, I’m relatively new to coding and Flutter. I am looking at creating something that is able to detect the value of a user input and display the results.
For example if a user was to input 2 values on axis x and y, the code is able to create a line the height and width of the input.
I hope I am making sense here and I am not looking for the direct code, more of a point in the direct of a solution.