How to define varying size of inputs for codegen?

I'd like to make mex file executable on GPU and hence am studying the "GPU Code Generation: The Mandelbrot Set" demo. The problem is how to properly define the varying size of the inputs, i.e. maxIterations,xGrid and yGrid of the function mandelbrot_count when calling codegen:

codegen -config cfg -args {maxIterations,xGrid,yGrid} mandelbrot_count

In the demo example the inputs size is fixed and if you change the inputs sizes, the mandelbrot_count_mex throws an error. I did a research and found the function coder.typeof to define the type of inputs. So, when defining, e.g. xGrid = coder.typeof(0, [inf inf]) which defines both dimensions of the xGrid as varying, then it solves my problem but the code becomes much slower then when the input size is defined as fixed. Is there any other solution to the problem please?