If you on a 1280x800 surface have a 250x375 frame, then the calculate the proportions like so:
250 / 1280 = 0.195
375 / 800 = 0.469

Now you'd just set the frames width to 0.195 * Me.graphics.GraphicsDevice.ViewPort.Width and the height to 0.469 * Me.graphics.GraphicsDevice.ViewPort.Height.

The X and Y coordinates would be done in the same way:
200 / 1280 = 0.156
200 / 800 = 0.25

Set the X coordinate to 0.156 * Me.graphics.GraphicsDevice.ViewPort.Width and the Y coordinate to 0.25 * Me.graphics.GraphicsDevice.ViewPort.Width.

I think thats the best way of going about it, altough I'm relatively fresh with XNA aswell.