https://www.walter-fendt.de/html5/phen/doubleslit_en.htm
This is a nice applet for you to get a feel of how the wavelength and slit separation affect the spacing of the fringes.
The applet allows you to set the slit separation to as narrow as 500 nm. In practice, the slit separation for a typical double slit is usually close to 0.1 mm.
xmPhysics 