The units of png and jpg are always measured in pixels. In Photoshop you can edit your settings to pixels. (Edit > Preferences > Units & Rulers)
Scaling of images can be optimised by enable mipmap. Mipmaps are smaller copies of the original, making rendering smaller variants faster. Mipmaps will be recreated every time you create a texture with dxCreateTexture (by default mipmaps are enabled, see argument mipmaps).