Bitmap Size???

Help me please

what is the best size for a bitmap with a NSIS installer
(just like the Examples\gfx.nsi)

you can resize them, as you can read here. for the default sizes, check NSIS\Contrib\Graphics.

To can add images of any size to NSIS user interfaces.

If you are talking about standard Modern UI bitmaps, see the Modern UI Readme.