Ok, I've got this method in my gui class that extends InventoryEffectRenderer:

@Override
protected void drawGuiContainerBackgroundLayer(float var1, int var2, int var3) {
	GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F);
	mc.getTextureManager().bindTexture(TEXTURE);
	int x, y;
	x = (int) (((double) 8 / 854) * mc.displayWidth);
	y = (int) (((double) 8 / 480) * mc.displayHeight);
	this.drawTexturedModalRect(x, y, 0, 0, 175, ySize);
	x = (int) (((double) 388 / 854) * mc.displayWidth);
	this.drawTexturedModalRect(x, y, 175, 0, 31, ySize);
}

 

For some reason, the gui doesn't properly scale itself on high resolutions, I haven't tried lower resolutions yet.

 

Here's at testing size:

 

And here's at a higher size:

That's because you are using the Minecraft display height and width directly; you need to create and use a Scaled Resolution:

ScaledResolution resolution = new ScaledResolution(mc.gameSettings, mc.displayWidth, mc.displayHeight);
resolution.getScaledWidth() // there is also a double version, if needed
resolution.getScaledHeight() // there is also a double version, if needed