First, here is a trick to see the actual space used by the plot:
Grid[{{Plot[Sqrt[100 - x^2], {x, 0, 100}, PlotRange -> Automatic,
ImageSize -> {250, 100}]}}, Frame -> All]

Grid[{{Plot[Sqrt[100 - x^2], {x, 0, 100}, PlotRange -> Automatic,
ImageSize -> {500, 100}]}}, Frame -> All]

So you can see that it did actually use more space (width). Now if you want to widen your plot, this is something else. You can use Aspect ratio for this.
Grid[{{Plot[Sqrt[100 - x^2], {x, 0, 100}, PlotRange -> Automatic,
ImageSize -> {500, 100}, AspectRatio -> .2]}}, Frame -> All]

But this does not look too good.
ps. (You can also use Framed@Plot[...]
to see the actual space used instead of Grid as above)