I draw cardmarker with original size of texture with a little code:
def create_sprite(filename, x, z, screenWidth, screenHeight, transparent=1):
tex = loader.loadTexture(filename)
cm = CardMaker('spritesMaker')
sprite = NodePath(cm.generate())
sprite.setTexture(tex)
#Scale and position
sx = float(tex.getXSize()) / screenWidth
sz = float(tex.getYSize()) / screenHeight
sprite.setScale(sx, 1.0, sz)
sprite.setPos(x, 0.0, z)
sprite.setTransparency(transparent)
return sprite