That comes down to your screens pixel pitch and viewing distance. At ANSO SuperScreens we will help you select the best product for your intended audience.
As a rule of thumb, for outdoor screens with individual Red Green & Blue LED's, your screens pixel pitch equates to how far away in metres your audience should be. i.e. A SuperScreen with a 12mm pixel pitch will look best from 12m or further away.
When it comes to indoor and semi-outdoor screens with Red Green & Blue in the one LED package (or SMD LED's), 3/4 of your pixel pitch will give you a comfortable viewing distance. i.e. A SuperScreen with a 5mm SMD pixel pitch will look best from 3.75m or further away.