This function is designed to work with the xyrel argument in the main plotting functions.
It returns "f" (for a fixed aspect ratio of one) or "m" (for maximized).
If the ratio between the x-size and the y-size is between 0.1 and 10, it will return "f".
Otherwise, it returns "m".
If both x any y are missing, the "size" refers to the dimensions of the matrix.
(So, returns "f" for up to ten times more columns than rows, or vice versa).
If both x and y are supplied, the "size" refers to xlim and ylim, computed from the ranges of x and y.
If one is supplied but the other is not, then "m" is returned.