You can manually specify the plot range.
a = 5;
Plot[Sin[x], {x, -a, a}, PlotRange -> {{-a, a}, {-a, a}}]
Alternatively you can set the aspect ration of the graph.
Plot[Sin[x], {x, -a, a}, AspectRatio -> 1]
Not sure if this helps, could you be more speciifc?
Details and Options in the documentation of most functions include preset options; maybe there's something I missed in there that could be useful?
Good Luck.
